[FirestormX]

I can see both sides. Personally it would annoy me to have someone yell "grenade" and it end up not being a grenade. For the sake of keeping the game fun, I usually treat a grenade detonation as something that will clear the room I'm in, regardless of if I somehow don't get hit with a BB. So if someone says they threw a grenade in, I call myself out if I can't get to cover. It makes it more fun for the other side - they had a plan to clear a room with a grenade, someone used the grenade that they spent all that money on, and they expect the room to be clear. I'll resist within reason (eg dive for cover and stuff), but I try to play along with the idea of keeping the game fun for the other side as well.

Granted, most of this happens at UA, so calling myself out usually just means walking 15 seconds back to spawn, but it has that "feeling" of someone yelling out "I hit you!", and you just calling the hit to avoid a hassle.
If someone calls "grenade" and something goes flying into my room, I know that I'm going to call myself out regardless, and it's kind of a piss off to realize that my "trying to be nice" was taken advantage of.

I'm not saying it should be universally discouraged, I'm just pointing out that it can be abused against more trusting players like myself. =P

Edit: Also, I'm talking about small, friendly, skirmish games. More long-term games, with large teams, large fields, etc, I'm not as "casual" about giving myself up to someone randomly yelling "grenade" near me.