Typically, grenades don't expel bbs at quite the energy as our guns do, so the safety issue regarding blindfiring isn't as prominent. So no.
The second scenario you're describing is called prefiring. As long as you have a direct line of vision connecting your barrel to where you're shooting, it's not blindfiring. Prefiring's a perfectly valid tactic, although it really is kind of speedsoft-like as opposed to proper milsim techniques that enforce proper trigger and fire discipline (limited amounts of ammo).
There's another thread (
http://airsoftcanada.com/showthread....ighlight=blind), where we go over ad nauseum what's considered blind firing.
Hope this helps.