Being someone who uses a suicide button, I don't think it's griefing because it's still moving the round along, and the prop is still dying. Granted I only pinpoint my suicide on certain people (here's looking at you elite skilled Exile and HOW THE HELL DID HE HIT ME WITH THAT FLARE, GOD FUUU- Glaceon)
Just use the mentality "I made you commit suicide, so that means I killed you"
As for dispenser blocking, the engineer can't keep up with you and sometimes he has that squirrel vision where shooting everything but you happens. The dispenser helps even out his inability to chase you. If he can't catch you, he can sure as hell block you. It's annoying sure, but for the most part I do pretty well staying hidden in spots from the times I played prop hunt