It is definitely the safest approach. I suspect that it isn't done very frequently but you are concerned about it and if you don't follow through I think that you will worry for a long time after, and that wait to really be sure is measured in years, not weeks or months with today's diseases. I say, go ahead and ask. The other person can say no, yes, or try to talk you out of it.
If the person says anything but yes, you will have a bigger dilemma to deal with but not nearly as serious as dealing with a serious STD. Some people are concerned that their employer who provides their insurance coverage will find out that they were tested and suspect that they already have a disease.