Charlie
Make sure you haven't sawn twist into the tenon before you start trimming. Set a decent stainless bench rule on each face of the tenon and sight it against the tenon shoulder. Check in the middle of the face of the tenon and near the end, checking both tenon faces.

If it tests wonky, then test the component for overall twist by putting a regular winding stick on the tenon and then one near the other end of the part. If the part has been sitting around, it may have become twisted. You may have to fix the twist by trimming the tenon down pretty substantially and then packing it back out again with veneer once it's out of twist in order to get a snug fit in the mortise.