I mindread everyone. Near the end of the D2:ED section of DKS I maxed the Mindread skill (went from level 1 to 15), so after that they were free (though there are places in the FoV section where opponents respawn, so experience debt is pretty easy to get out of).
It is a little better to mindread first and then clear opponents, leaving quests until last. If you mindread first you will stay at a lower level longer, which gives more experience from opponents. Quest experience is fixed, so you are a little better off delaying turning them in (though it really wouldn't hurt if you wanted to finish quests as soon as possible to keep your quest log from getting cluttered, etc).
Wisdom / Mindreading - worth it? If you ignore the Mindread and Wisdom skills, reload for 'useless' mindreads, delay turning in quests, etc it will make a minor difference in how fast you level (more so at low levels), but by the end of the game it really isn't going to make that much of a difference. There is a benefit from trying to maximize experience, but not enough to worry about if that doesn't fit your playstyle, or you run into trouble with opponents and want to turn in a few quests to level, etc.
In the original release of D2:ED I delayed turning in some quests, but didn't really go out of my way trying to level. I used the Talisman of the North (+2 Wisdom bonus) for quite awhile, and between about level 13 and the Hall of Echoes got Wisdom up to level 9 or 10, 4 or 5 of which was from equipment bonuses (equipping Rothman's Bow to turn in quests). Before the Hall of Echoes I boosted it to level 13, to see if I could get to level 37 before the end of the game, thinking the area might be larger than it was. I reloaded for some useless mindreads, but not all.
In DKS I specifically tried to do all the mindreads first, then clear opponents as much as possible and then turn in quests, to maximize the experience given. I didn't learn Wisdom, though did eventually collect jewellery with a combined +8 bonus. I deliberately mindread pretty much everyone.
Neglecting the balancing and Sentinel Island mindread differences between D2:ED and DKS (I was at a higher level in DKS, so got more of an experience bonus), at best (assuming only a modest increase in quest and opponent experience levels) my efforts brought me from level 36 and 3/4 to 37 and 1/3. Considering I was only on level 37 for maybe the last 10 minutes of the game, that isn't a huge difference.