Just don't neglect doing an NVRAM reset (from Tomato) after switching, or before switching back to DDWRT. Dunno how important that really is, but it all works smoothly for me (WRT54GL) when I do it that way.
And if starting from scratch, with Linksys factory firmware on the router, then the initial install of Tomato is as simple as clicking the "Firmware Upgrade" link there, too.
In fact, that's probably the easiest way to install DDWRT from scratch: use Linksys Firmware Upgrade to first install Tomato, and then.. procrastinate.
Cheers