I've tried using a poncho in both orientations.
straight gives you more cover from rain, but alot of the tarp is not actualy doing anything, and is loose weight. it also needs 4 guys to be pegged out (which means you need more pegs, for the ultralightweight guys)
plus you need a bigger tarp to cover the length of the hammock
diagonal only needs 2 pegs (lighter, quicker to set up) covers the hammock better lengthwise, but doesn't cover you very well from the rain, I ended up with my head getting dripped on (but I sleep diagonaly in the hammock, not straight)
best comprimise I've seen was a hexagonal tarp, which eliminates the useless tarp area only protecting ground from the rain, but it does still need 4 pegs.
I just got a big groundsheet from millets, heavy but it covers everything, and was cheap.
one day I'll get something lighter, but for the minuit it does the job (I don't really do "fast and light" anyway)
I have fallen out of a hammock a few times, but only because I was doing something stupid. like half hanging out to get something from my bag, wrapping myself up in it and hanging upside-down, or trying to support myself with it wrapped up "warriors hammock" style. the stupidity of bored explorer scouts never cease to amaze......