As a first note, I will provide the obvious statement that this may vary from jurisdiction to jurisdiction and I speak only from knowledge of UK law and the resources at my disposal that may cover some other jurisdictions in a light touch manner.
As your question considers the scenario where you are spoofing a check for a paid account, I will assume that you do not possess the appropriate license for the software and are using this technique to circumvent what is a license verification procedure.
That said, usually this would be illegal as you are using a piece of software without the appropriate license. This follows the agreement (read contract) you have formed in the installation of the program - the EULA (end user license agreement). Where the EULA stipulates that you only have rights to utilise the software if you have the appropriate license, you would still be in breach. See section 1.g. of the McAfee EULA.
Furthermore, while you may not be in direct breach of the decompilation/modification restrictions imposed by the EULA (3.f.), you may still be in breach of the proscription of 'Reverse Engineering', as through your analysis of the software you have been able to exploit a particular aspect.
Obviously, I have opted to use a fairly comprehensive EULA for the purpose of this answer and it is feasible that there may be software produced without the relevant provisions and protections.