Depends on how old your eyes are
And your accuracy requirements. Scopes don't necessarily help you shoot significantly more accruately. They help you see the target. A scope is just as useful at 25 yards as at 250.
With good quality target type iron sights like on the Ruger, or on an AR you should be able to hit human size targets out to at least 200-300 yards. The USMC trains at 500 yards with iron sighted AR's.
As long as I've got good light and a visible target there isn't a huge difference in group size out to 200 yards. As far as I've shot with irons.
Scopes come in handy when shooting in poor light because they help you see the target better. As long as you stay with something with a fairly low power and long eye relief scopes are proving faster for target aquisition in rapid fire shooting.
I'd zero at 100 yards. You won't be off much at all at 50 yards and still be close enough out to 200-300 to require very little hold over.