Snug, not much past finger tight. Just remember the thread locker if the mounts are metal. Also watch how much thread locker you use, you could melt plastic especially if they are the T-Pro MG90S servos.
I tighten them enough to compress the rubber mounting grommets a little bit (little bit of bulging). If you're using the brass collars inside the grommets then those should prevent you from over tightening.
