Doctor, should we wait for the sinus tract to close before filling?

It is not essential, but waiting is the more cautious approach. I used to treat more such cases in one visit; now I generally apply CaOH2 and wait at least three weeks, completing treatment after seeing the fistula heal. If in some cases it remains open despite several medication sessions, I fill the tooth with MTA and follow it again. If it still does not close, I prefer apical surgery.