I use subtitle workshop (freeware). It is a bit picky about which video it accepts and it is also a bit buggy at times, so one needs to get used to it, but it has provided me with great subs all the time. I save in sonic scenarist mode (ssa), because DVD LAB PRO can perfectly handle that. My version of DVD LAB PRO has difficulties with the borders for srt subs, which meant that sometimes the lines were stretching over the visible area, so no one could read them entirely.
What I always do is: I convert my finished version to avi and then start with the subtitles. Usually I alter the existing ones, by moving them to the right timecode, but sometimes I need to add some lines. Subtitle workshop can do all of that.