on a child or very elderly person?? probably wouldn't be the best idea, not for a routine treatment anyway.
On a middle aged person then maybe you could, but try to avoid them wherever possible, especially GA.
I did end up having to sedate someone in oral surgery a few weeks ago so it can be done but more often than not you should only try to do either of the two when the patient has a clean (relatively) bill of health.
As junglemonkey mentioned already-
child---> tell, show, do. Always demonstrate that it doesnt hurt. Use children linguo, for example if you are going to drill decay out of their tooth say something like, "this thing here will zap all the bugs out of your tooth" etc. Stickers always helped as well (with me anyway lol!)
with adults, the "if you want me to stop, raise your left arm" trick always has to be done, and if they feel uncomfortable in the slightest, especially the first time, do stop. They often do the first one on purpose to see whether you are telling the truth or not. Also, informed treatment is important- tell them everything you are doing, and why. It distracts them from the phobia. Obviously when you tell them what you are doing, avoid using words such as needle, drill etc as it will not help them one bit.