AFAIK, this is true. Unless the deck has a switch that specifically allows you to output from different heads, you would probably be hearing the original source, not monitoring from tape. Multi-track decks allow you to monitor from the record head (for obvious reasons - like so overdubs line up with what is already recorded), but the quality is not as good as when you switch back to listening from the playback head like you would for mixdown. The playback head is delayed slightly from the record head since the tape hits them at different times.
Doing what you want to do is difficult. I guess you could record to tape, dump to digital and record overdubs to tape while listening to the already-recorded stuff, then dump to digital again, then line everything up before the next overdub.
You could also record everyting digital and send the whole mix to tape and back. Tracking to tape first sounds better to me, but it is substantially more complicated when you need more tracks than you have on the tape machine. You really have to listen and decide if the extra pass through the converters and the possible unwanted artifacts of a consumer-grade tape deck degrade the signal more than the intended benefit of tape (which I love, don't get me wrong). That's one of the reasons going tape first is better - only one trip through the converters.
There's some really smart tape folks in the analog forum here, btw.
