Should I let RS do my dns or use my server to do DNS? I have let RS do it in the past. Less hassle in my opinion. Am I wrong?

If RS handle it you have to manually setup every domains DNS. When Plesk does the DNS for you it's 100% managed and your customers can make changes from within the control panel (e.g. adding MX records etc.)

I'd say it's best doing it yourself - RS may be useful for some but DIY is always better.

Hope this helps.

I wrote a guide for this. It should make the change a breeze

Jaredweb.com Plesk Nameserver Gudie