Does a NIOS II need reset?

It doesn't take long for the PLL to lock into phase. Off the top of my head I think the delay in the reference designs is at least two times longer then the PLL locking time so look at the counter value in that one to see what a good value is. The counter should work fine if you use your osciallator and not the PLL output to clock it.

But like what was said before, an external reset is the way to go (just like a bomb with an off switch is a good idea too, lol). Really you are shooting yourself in the foot if you do not have an external reset, you could end up costing you or your company a lot of money if they have to re-spin a board to put one in (and it helps you debug too).

Cheers