I ran the outs from my TASCAM 428 back into my mixer as well, and I never used the tape returns, I always went directly into 2 channel line inputs. Now that I have switched to the Echo Layla, I am also running the outs from that system directly into a few line-ins on the board. I never tried the tape returns, but since my goal is to blend the outs from the computer, with the new material that is being recorded (I can monitor the signal at the same time as I send it to the computer), I like having the signal with an easy-to-adjust fader. When I was using the 428, I just split the signal (Y-cable) from the output of my mic preamp (I never used the 428's pres), one send to the computer (428) and one to the mixer for monitoring.
If you are just using the mixer for monitoring, I would think that the tape ins would work just fine, and would save you a channel on your desk. The quality of the playback might differ from mixer to mixer though, and you can bet that the line-ins on each channel would be better or the same than the tape-ins, certainly not worse. On the other hand, the tape-ins would *never* be better than the line-ins...it just depends on the board, and how many free channels you have.