You should mind the source material: If your game uses Adlib or GUS music, you will profit from every Hz you use (the average human should hear the difference between 22050Hz and 44100Hz, but not between 44100 and 48000Hz, so you're best off with 44100, and if your box is too slow, you could try 32000 as a compromise). If there's CDAudio, you either use a real CD (then DOSBox settings don't matter) or a CD image, then 44100 is full quality - nothing can be gained by going even higher. If it's MIDI, DOSBox settings don't matter. For digital sound effects, the best mixer rate is dependent on the game. Games earlier than '95 probably won't have samples with more than 22050, so you won't gain anything with more. Around '90, 11025 was usual (DOOM for example still had 11025Hz samples IIRC).
Bottom line: Use 44100 if your machine is fast enough and you really care, otherwise 22050 is a good value as well. Go for 32000 if your box is a little too slow at 44100.