it all depends on what exactly you overwriting

 

I was wondering the same thing. i just now started to get comfortable installing themes and roms as well. Im using the 928s black glass theme that i got when i deodexed, the black task bar, and Fabs "goodies". If i wanted to install REVO or ApeX do i need to uninstall anything or can i just drive right into another theme?

 

I use titanium back up to uninstall apps. The paid version allows you to freeze/defrost apps. You're best bet, if you want to try new roms or themes is to make a backup. Then if you don't like it you can just use your back up to get back to normal. When I got the OTA I rooted and made a back up. If I want to try a new Rom I just load that, flash the Rom, put my theme over ut and see how I like it. If I don't resort back to my current set up and everything is great.

 

you can go over it but then you are running the risk of leaving remnants from roms or themes you have gone over into your new rom ,,,,,really with themes its not too big of a deal but when it comes to roms its always best to start fresh ...your best bet is just after the point of root and installing clockwork and bootstrap and all of your rom required apps before flashing anything you should make a backup that way you always have a fresh start point for different roms...also quite a few roms come pre deodex eliminating the need for 928