I agree with Minwah. Setup clrmame and after that it's easy to check and fix roms. Clrmame can even automatically check for and upgrade to new versions of clrmame.
When a new mame comes out, just get the data in a dat file or by pointing clrmame to the new mame and making it lookup the info, and scan.
Then drop any new/fix files/zips into the scan missing windows and clrmame will put the correct files in the correct zips if included in the stuff you dropped.
Don't even worry about the "Rebuilder" and "Merger" buttons if all you want to do is keep up to date; just use "Scanner".
Is there any that auto grab missing ones???
Clrmame would do it, I think, but you need a site that lets links from the outside download files. Read the help file for more info.
Romcenter is supposed to be easier, but I found clrmame so easy I stopped using romcenter.