Yeah, this is much more likely to happen if there's actually some meat to take out of your side project, so most devs won't have to worry about it. Developers should understand ahead of time that asking for your boss's word isn't going to mean anything if the side project takes off (for multiple reasons).

However, in that case, a legal fight may be somewhat tenable (though it always sucks for everyone (except lawyers)). If you got them to sign a document releasing the project, you'll be able to win that much more easily.

If you can get the signature without too much hassle, it's worth it. If you can't, it's not worth ever mentioning it, IMO.