If your party is strong enough, the best place is the outer courtyards of Valjevo castle, the random encounters with giant patrols there (when the alarm is ringing) give tons of gold & XP (3304XP) and a longsword +1. Unfortunately you'll probably already have to be quite high level already.

(Although, I have to say that in my opinion grinding is pretty useless in this game as gold has only very limited uses, and all the characters will eventually reach maximum level anyway.)

It's just that I am using a party with particularly heavy multiclassing: 2 elf fighter/magic-users, 1 half-elf cleric/fighter/magic-user, and 1 elf fighter/magic-user/thief, plus two single class characters: 1 human cleric and 1 human magic-user. (I actually went against the advice I read and substituted the single class magic-user for another fighter/magic-user, as I was concerned about not having a magic-user advance quickly enough. It also allowed me to have an extra female character without worrying about strength limitation issues, though that is just a personal thing.)

I am still concerned that my triple-classed thief in particular won't be high enough level to deal with traps as they come along. I know that traps in this game tend to be of the mild variety (usually), but I would still like to deal with them. Even without that, all my fighters will advance slowly. This is why I am particularly interested in grinding before I reach Valjevo Castle, which generally is the last place you go in the game.

If you leave Sokol Keep and return, the number of random encounters are reset to 4.

If you haven't completed the Temple of Bane yet, there are unlimited random encounters on that map and in the adjacent Wealthy Area.

After you accept the mission for the auction at Podol Plaza, there are unlimited number of random encounters there as well.

True, but in the Podol Plaza, if you get discovered and lose your disguise, it becomes impossible to complete the Phlan council's mission OR to clear the block. Is it possible to clear the block after carrying out the auction mission and returning to the council for the mission reward? I seem to recall this being a problem on some previous playthroughs (i.e. after the auction mission was completed, all encounters in the block simply disappeared).

On the whole, Sokal Keep might be good for a little grinding at low levels, but you won't get much experience from four patrols of skeletons and zombies. The Wealthy Area and Temple of Bane sound more promising. Podol Plaza is out, because I am strongly into completing all missions, and if possible getting all bonuses from the council.

The wilderness still sounds like the best place to grind, once you are of sufficient level to be able to handle it (though as a noted previously, the fights there can vary radically in difficulty).

I think the encounters in Podol Plaza still occur, even after witnessing the auction (but I'm not 100% sure).

Of course there is also the tavern in Phlan to grind encounters, or the city guard (but if you fight the guards you will need to complete a commission in order to regaining access to Phlan shops).