I have not used Transcribe but I do use the 1/2 speed function offered here quite often. I personally have not had an issue with it ruining my ears. My ears, and probably yours as well are tuned into the bass line so I know when to make my chord transitions.

In terms of how often or what the duration should be, I think you need to do what you need to do to get the parts down. This function is only a means to an end.