Ammo bonuses (the basic +1 through +5 bonus) should stack with bow bonuses to make the majority of basic magic arrows and bolts at least mildly useful instead of useless. Ammo that also does extra damage is already useful, of course, even if its base bonus isn't, but "vanilla" magic arrows and bolts rarely are.
Given that +1 bows are exceeding easy for even low level characters to get hold of, all the +1 arrows in the game may as well be normal arrows. And the same holds true for +2 and +3 in most cases. Even +4 and +5 are rarely of use. By the time you have access to the higher bonuses, you'll almost certainly have a better bow. And even if you sacrificed a couple of pluses for some other effect, the measly +1 or +2 addition you get from +4 and +5 arrows barely seems worth it.
Considering that most people seem to feel ranged combat is, if anything, a little underpowered, allowing totals up to +10 (+5 ammo, +5 bow) is hardly likely to upset the balance.