Is this legal? Is it legal to count to the minute without rounding?
Yes. It is legal as long as they comply with minimum wage laws. Usually it is not recommended because if they don't round they might be underpaying and subject to labour investigations, but they can do it properly and legally.
Must they tell me how they round, for example round to the nearest 1 minute, 5 minutes, 10 minutes etc?
You have a right to a proper wage statement. It must indicate the hours worked in a way that allow you to calculate your wage from the hourly rate. Note that they cannot round down your hours (an 11-minute interval must be paid for 11 minutes, 15 minutes or 20 minutes if they round to 1, 5 to 10 minutes).
Normally the closing procedures don't take a full 30 minutes and I was trained to leave as soon as they were finished.
It seems to me the employer agreed to provide me with an 8 hour shift, would it be within my rights to wait until the last minute to clock out even if the closing procedures were finished?
BC's minimum hours law is outlined here in the ESA Guide.
A shift in BC must be paid a minimum of two hours.
If you are scheduled for less than 8 hours and you report to work, the employer must pay you for 2 hours of work at minimum. If you work for over two hours, actual hours paid is enough. If you report to work at 8:30 pm, they would need to pay you at least until 10:30pm.
If you are scheduled for eight hours or more, 4-hour pay is the minimum unless there is an exceptional circumstance beyond the employer's control. If you are sent home early almost at the end of your 8-hour shift, you likely do not qualify.
Of course, if you requested to leave early, or if you are sent home because you are not able or fit to work (due to illness, alcohol, failure to follow safety instructions etc.), the employer does not have to pay. Whether clocking out because you are trained to do so counts as an request from yourself or your employer could be tricky to determine, but it is on the employer to prove it is a voluntary request. ESA Guide says
The employer must be able to show that the employee initiated the request to leave work early, and that the employee was not merely accommodating a request made by the employer.
It is not your responsibility to promptly leave early, so you can wait until the last minute if the employer doesn't ask you to leave. The employer however can send you home earlier.
Unfortunately, if you insist on taking the whole shift, you may face retaliation (e.g. scheduling you for less hours).
It would be probably the best for you to consult a lawyer or a union to see if regularly sending employees home early may be excessive for other reasons.
If you are already unionized, tell your steward and discuss an appropriate solution; many collective agreements for retail in BC contain additional provision regarding minimum shift pay (but usually do not require to be paid the whole hours scheduled). If not, well, you have something to think about, in any case many unions would be happy to provide labour advices even if you are not unionized.