So is there any drawback to assigning drive letters A and B in Windows 7?
The only drawback would be older software that assumes that means you have a floppy-disk inserted and then further assumes that something should be done with that floppy disk (or just functions differently if a floppy disk seems to be there)
So unless you run MS-DOS software in windows compatibility mode or something (and why do that? DOSBox FTW!), or perhaps older 16-bit windows software (which won't run on 64bit win 7 anyway), I really don't see how this could ever even be an issue.
Not that it's all that likely if you do routinely use such software.