today i noticed that if i arch my back as much as possible and allow my hips to leave the bench I could bench press more…
is this poor form/cheating or am i allowed to do this? 
Your hips need to remain on the bench. That’s cheating.
poor form that im guessing can result to an injury.
cheating doesnt matter that much imo, if you use more weight, your chest and triceps will get worked as much as before, and the extra weight will be used by whatever you use to cheat. problem is, you may get injuries, without really getting more of a workout, just a higher number
Equate this to decline benching. This is the effect you are trying to simulate. Not only does this “technique” look ridiculous and flag you as a dork, it is poor form and could be potentially dangerous with heavy weight.
I asked this some time ago already.
Good replies (read the linked article as well):
http://www.T-Nation.com/tmagnum/readTopic.do?id=1817590