I used to never do that in BIAB, but export the MID, import it into Sonar, and do the ritardando there.

I think that is more problematic with RealTracks though. For what you export and import with a Real Track there is audio, not midi.

Certainly there are tools in Sonar and other DAWs to stretch time without changing pitch, etc. Doesn't always work well though, especially if it is a major tempo change.

Stretching Real Tracks would I assume work better in BIAB, as it has time-stretching feature made especially for the Real Tracks.

So yes, now with RT, I think BIAB should include that capability. And changing only at the beginning of each measure does not cut it.

How about RealBand? Does it stretch the RTs well enough to do accelerando and ritardando with it?

In Sonar there are various ways to do a ritardando, including drawing the tempo map, etc. They way I always found easiest (for MIDI only though) is the command "insert series of tempos". You specify the beginning time of the ritardando, and what the beginning tempo should be there. Then you specify the end time of the ritardando, and what the tempo should be there. You also specify whether you want the tempo changes to occur every beat or every half beat. Click OK and it figures out all the tempo changes necessary.