Read their TOS carefully. In most cases, they don't bar ANY forwarding or redirects - only those that have some intermediate display (pop-up, page that flashs briefly, etc.) and/or those that are done for the purpose of gaming the system. (Such as having the same store display two ads on the same page.)
As an example, Google Adwords permits redirects, as long as the landing page domain is the same as the display URL domain. (And no pop-ups, intermediate pages, etc.)
Other uses of redirects are in most cases legitimate. For example, many sites use a third-party metrics site to collect statistics on their PPC traffic. The request first goes to the metrics site, which then redirects to the advertiser's website. Most/all PPC TOS's permit this.
Getting back to the main subject - I don't recommend doing ANY kind of redirect with your registrar. This is just one more potential point of failure. Much as registrars may try to hide this fact, redirects have NOTHING to do with DNS. The registrar is simply running a webserver (probably NOT the same hardware as the DNS servers) which is redirecting to your site. One more thing to potentially fail. And if you've carefully selected your web host to have the best connectivity for your user base - that is out the window, since they are being first bounced off of your registrar's redirect server, which may or may not be optimally located on the net for your needs.
The best solution is to use a web host that allows you to have unlimited virtual domains (or at least enough for your purpose) and do the redirects yourself. You shorten the failure path by one server, and get to put the server where YOU want to put the server.