CRC vandals have gone as far as hacking hidden intros into roms and reverse the CRC back to the proper dump. So I think it's worth worrying.
Are you serious!? Here I always thought to myself there was no real reason to use anything beyond CRC32 because the chances of an actual (as in not intentional) collision are practically 0 and I didn't think anyone would be enough of a jack ass to do it on purpose (silly me :|).
I never used to collect SHA-1 for the DB either, but a few people requested it some time ago so I added it anyways. FYI, you can view the SHA-1 by hovering over the the CRC32 on the website. Not terribly useful like that, but it's there
As long as the mapper bank-switching info is known and correct, yes it is. If not, this mis-information gets built into the dumping program and in turn the banks will come out in the wrong order. If emulators also use this bad info for implementation, then you really have a problem because the game will appear to run fine because the errors cancel each other out. I've been keeping a close eye on this though and I don't believe anything in the DB is "dirty" like this.