It's probably fine. Back in the day (early 00s) I used Gentoo and I think pretty much everyone used march (because why not?) with no real negative side effects. It may be a little more buggy with uncommon or new architectures.
I did have the occasional weird breakage with -Os, though (once it broke make...). Anyways, -O2, -O3, and -Os should be pretty reliable as I imagine they are the most used optimization flags these days.
You generally don't have to tweak options manually unless a small increase in performance is very important to your application or you are running specialized programs that will benefit largely from a specific optimization. Remember -- most Linux distributions use pretty basic cflags and are reasonably performant.
Come to think of it, the Gentoo project has probably been useful in rooting out weird cflag bugs in GCC. :)
I did have the occasional weird breakage with -Os, though (once it broke make...). Anyways, -O2, -O3, and -Os should be pretty reliable as I imagine they are the most used optimization flags these days.
You generally don't have to tweak options manually unless a small increase in performance is very important to your application or you are running specialized programs that will benefit largely from a specific optimization. Remember -- most Linux distributions use pretty basic cflags and are reasonably performant.
Come to think of it, the Gentoo project has probably been useful in rooting out weird cflag bugs in GCC. :)
EDIT: Also check this out:
http://wiki.gentoo.org/wiki/Safe_CFLAGS