Depends on the scenario... if I feel like he/she really needs that extra sleep, and it doesn't impede on anything we are doing, I will let them sleep. I will wake up a sleeper from morning nap, for 2 reasons. a) we have a kindergarten school pick up to do b) it usually means a tougher time getting them down for afternoon nap, which I "need" more!
Here, quiet time is from 1 (or just after lunch) until 3. Anyway you can put this little one down a little earlier than the rest, if you feel like the extra time is needed?