I just leave it at 1 bit, yeah.
Now, whether the mixdown dither option has any effect on the 32 to 24 bit conversion, I'm not sure, but I check it just in case. Unless it's very stupid, it probably won't dither if it doesn't have to! However, the dither option under properties does apply, as far as I know, to the internal 32 to lower bit depth conversions.
Slackmaster 2000