In general, wash outer-wear inside out, undergarments right side out.
The greatest amount of wear-and-tear for most clothing actually happens in the dryer, and the second-greatest in the washer. This is entirely logical of course; a pile of clothes are tumbled around in close proximity to each other. Of course their surfaces tend to abrade each other!
Now, when you are wearing something the side which gets the most wear-and-tear is the outside. For outer garments – e.g. jeans and t-shirts – they will last longer and stay looking fresher if you can have the inside surface take on more of the rubbing and scuffing that happens during laundering.
For undergarments, though, you are generally less concerned with how it looks and more about how it feels. That satiny-smooth finish on the inside may not last as well if it takes too much abuse, so maybe better to leave it only rub up against a satiny-smooth finish. For the same reason, strongly consider using a delicate’s bag, especially for long or stretchy things like nylons or bras which might tangle up in knots.
A couple important notes: the side facing out when in the washer also gets more physical cleaning of the surface, so maybe leave grass-stained knees and chocolate-sauced sleeves out (you did remove all the surface crud and treat with a spot-treatment first, right?) Also, heat is a serious issue for fabrics which use certain synthetics, especially stretchy fabrics and derivatives of nylon (I’m looking at you, spandex.) Try to avoid putting these in the dryer; hang woven fabrics and fine knits; heavier knits lay flat to dry.