I've read where we're not supposed to reuse tapes.

HOWEVER, does that apply to DVCAM tapes that are allegedly made of heartier materials? Because of their added expense, I only plan to reuse the DVCAM tapes maybe three times each.

Any feedback?

It implies to all tapes for video. Including VHS tapes and whatever.
If you want to be professional do not re-use tape, unless for
testing purposes. The risk is just too great that you picked the
wrong tape and overwrite something import or the previous
magnetic signal is not completely wiped and you end up with
a busted tape footage wise.

If you shoot commercially where someone else is paying always
use new tapes (you can charge them for this), tape is cheap.

If you shoot for yourself you can take the gamble if you want. I
know I ain't (I don't do commercial work, only for myself and I
just buy new tape).

MUST I use DVCAM tape in my PD 170 to shoot in DVCAM mode or may I use a good regular non-DVCAM SONY tape to shoot in DVCAM mode?

I'm sorry, but I can't answer that question. I have no idea. I do
know that in DVCAM mode the tape runs 1.5x faster through the
camera (hence the recording time of only 40 minutes). I also know
that most people just use the camera in regular DV mode.

See the thread below:

http://www.dvinfo.net/conf/showthread.php?s=&threadid=24713

A DVCAM tape might be build to more stringent requirements to
make sure it won't snap (or something) at the higher recording
speed, but I doubt much else will be different.

Keep in mind that there is absolutely no difference in recorded
signal between these two modes. The only difference is that
each frame is recorded over a longer piece of tape and thus
reduces the chance of dropouts or other tape problems.