I can see where a sight would be needed for long-distance firing, and the phaser rifle from WNMHGB has a sight as well. But I would think for fairly short-range targeting, which is most of what we saw on TOS, they could get by on eyesight alone. Starfleet does train its crews in at least basic marksmanship, and I'd suspect that it's easier to aim and fire a phaser than it is a modern-day firearm since there are no recoil effects to throw off one's aim.