My rule is "tight enough to do the job, no more, no less.

On wheels you want the rear wheel not to slip (with horizontal dropouts) so a bit more tension might be needed than with a front. On seatposts, you want the post held tight enough not to slip or twist in normal riding - including hard bumps - but may want it loose enough that it twists in a crash possibly sparing you a broken pelvis.

An exact answer is impossible because bikes and QRs vary so much. Some people enjoy excellent clamping at very low QR tension, and others can barely keep a post from slipping even when closing with all their strength.

The guideline offered above of all the slack taken out halfway through the throw is a good starting place, and you can go from there, balancing good holding power and ease of closing and opening.