Depends on where you are and who you game with. At tourneys it is a big deal but with friends it shouldn't really matter at all.
Heh, then my firewarriors would throw you for a loop as they all have grenades on the model but have never actually had them in game. I just like the way they look . However, you are absolutely right about INFORMING the opponent. So long as you do so, there really shouldn't be a problem, especially in friendly games.Most people will go for a compromise. If someone (preferably someone wihtout access to wargear) in a squad has an 'everybody must take' upgrade like grenades, modeled on the mini then you can assume the whole squad has them.