I found it hard to change NPCs later along in BG. I had spent too much time with the characters. The party in my first BG game was composed of Imoen, Jaheira, Khalid, Minsc and Dynaheir. I kept the same party after the Gnoll stronghold bit.
Also, I always reloaded when one of my NPCs died, no matter what. I had to go through all the battles without losing a single character. Resurrection felt like cheating.