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PS. Always get the singular, plural and ing version of your site....I found out the hard way.
You can definitely run into trouble when different urls resolve to the same page -- even the basic "with and without www" variations of a single domain have caused me troubles. I know it shouldn't be that way, but lots of things shouldn't be.
This diluted PR for the princpal variation of the domain name, and in one case (on a second tier engine) all the listings vanished. So now I only purchase variations for protection and I usually just leave them parked. They only resolve if I have unique content to serve up.
Anyway, I still state that forwarding (not parking) won't hurt at all. You will not link to your net/biz/org sites and the SE would not consider this spam etc. I know many who have it this way. It didn't make a difference (good or bad) to their ranking.
even the basic "with and without www" variations of a single domain have caused me troubles.
I know of troubles when the hosting server lost control of the user, once they were directed to a different domain (SAME URL with/without www) - but for ranking issues?!?
Sometimes there's no obvious problem (except that PR is split and you are not aware that things could be better), and sometimes both addresses drop out of the SERPs.
I've also had sites where there seemed to be "no problem" - but in the past year especially, I have had others where there definitely was. So going forward I make sure that only one address accesses any given piece of data. I even try to avoid using "/directory/index.html" in my internal links, preferring to use "/directory/" in every case.
Yes, I'm getting obsessive. But I've been bitten more than once.