Small touch up FUE to missed spot on temples?

There is no absolute rule.  It is at the discretion of your doctor.  If there clearly is a blank spot, you can have a discussion with your doctor.  As long as there is a clear area where there were no grafts placed, it should not damage the transplanted grafts.

If you are sure that there is a missing spot it is possible to have a touch up after 90 days approximately.On the other hand yes there is always a certain risk of transplanting hair to a previous recipient area so soon.

Are you sure it was missed? Or did they run out of grafts? or did it get knocked out after?  If you are sure it was missed you just need to wait 6-8 weeks for the healing ideally

There is a risk, most certainly. That does not mean it can't be done, but it is not as simple as first glance. There of damaging the existing hairs that were there initially. They have had some trauma with the surgery and this is more trauma/injury. There is a risk of damaging the new grafts.  They have been working hard to survive and the effect of further injury right now is somewhat unknownTheir is a risk of poor growth as grafts are going into skin that is now in healing phase (there is inflammation there)There is a risk of asymmetry if things are a tiny bit swollen, it may be hard to know precisely where to place the graftsEverything has risk. The decision on whether to proceed or wait comes down to understanding the risks and benefits

You can do a touch up FUE after a week.  You do not need to wait weeks if its just a few grafts that need to be implanted. 

You can have a touch-up within 14 days after the hair transplant as long as there is no crusting present in the recipient area. Crusting causes infection so a clean wound both in the donor area and the recipient area is critical for determining the date you can undertake the touch-ups you want.