I was given a license plate with "In God we Trust" above the numbers. Later I learned that this was supposed to be a choice, and I was given none. Am I permitted to place a sticker over this slogan? I would not be covering up anything else.
In Florida, FS 320.061 says
A person may not apply or attach a substance, reflective matter, illuminated device, spray, coating, covering, or other material onto or around any license plate which interferes with the legibility, angular visibility, or detectability of any feature or detail on the license plate or interferes with the ability to record any feature or detail on the license plate
Delaware Code 2126(c)(1) semi-similarly says
No number plate, or any portion thereof, shall be covered with any tinted material, nor shall any other material be placed on or around a number plate which would conceal and/or obscure any information contained thereon, including the registration expiration sticker.
The first part of this, on a plain reading say "No, no portion", but the second part speaks of obscuring "information" – arguably (but this is not specified in the statute) a slogan is not "information".
Iowa Code 321.37 more narrowly restricts frames:
It is unlawful for the owner of a vehicle to place any frame around or over the registration plate which does not permit full view of all numerals and letters printed on the registration plate.
This suggests that it would be okay to cover artwork (but not a slogan, which is letters): but also it states the restriction in terms of frames, so one might assume it's legal to put tape over your license number (clearly not, and presumably the courts don't interpret the law that literally, still it's an interesting case).
No colored glass, colored plastic, bracket, holder, mounting, frame, or any other type of covering shall be placed, mounted, or installed on, around, or over any license plate if such glass, plastic, bracket, holder, mounting, frame, or other type of covering in any way alters or obscures (i) the alpha-numeric information, (ii) the color of the license plate, (iii) the name or abbreviated name of the state wherein the vehicle is registered, or (iv) any character or characters, decal, stamp, or other device indicating the month or year in which the vehicle's registration expires.
Because of the specificity of the Virginia code, the courts are more likely to take the wording seriously, so that you could obscure the plate partially, as long as you don't obscure the tabs, "information", or color of the plate.
The Texas case of State v. Johnson is interesting in terms of how such restrictions are interpreted. In one version of the law (section 502.409(a)(5)), the restriction was
A person commits an offense if the person attaches to or displays on a motor vehicle a number plate or registration insignia that...:
(5) has letters, numbers, or other identification marks that because of blurring matter are not plainly visible at all times during daylight;
(6) is a sticker, decal, or other insignia that is not authorized by law and that interferes with the readability of the letters or numbers on the plate; or
(7) has a coating, covering, or protective material that distorts angular visibility or detectability.
An earlier court (US v. Granado, cited in Johnson)
held that a license plate frame that partially obscures the name of the issuing state-but does not otherwise obscure the letters or numbers on the plate-does not violate section 502.409(a)(5)
Subsequently, the Texas Legislature amended the law to:
A person commits an offense if the person attaches to or displays on a motor vehicle a number plate or registration insignia that:
(5) has letters, numbers, or other identification marks that because of blurring or reflective matter are not plainly visible at all times during daylight;
(6) is a has an attached illuminated device or sticker, decal, emblem, or other insignia that is not authorized by law and that interferes with the readability of the letters or numbers on the plate or the name of the state in which the vehicle is registered; or
(7) has a coating, covering, or protective material that:
(A) distorts angular visibility or detectability; or
(B) alters or obscures the letters or numbers on the plate, the color of the plate, or another original design feature of the plate.
and in the case of Johnson, defendant had partially obscured artwork, which
alters or obscures the letters or numbers on the plate, the color of the plate, or another original design feature of the plate
and thus the court upheld the conviction, based on the revised wording of the law (even though the obscured material was not "vital information").
So: it matters exactly what the wording of the statute is in your state.
In Kentucky, the restriction is stated as
No rim, frame, or other covering around the plate shall in any way obscure or cover any lettering or decal on the plate
but this continues
except that, any owner who objects to the display of a trademark of a private corporation which appears on the registration plate shall be entitled to receive a set of decals from the county clerk in his county of residence to cover the trademark of the private corporation
(for a cost of $3). On the face of it, nothing can be obscured on the plate, except via the official decal provision which is limited to private corporation trademark displays. It's not clear what such a thing could be, unless it's possible under the special license statute. At any rate, the exception allowing a covering decal is very narrow, and requires a special government decal.