Well, to a certain degree each ROM will require some customization because of the differences from ROM to ROM. But that's really the reason WHY to try a new ROM, so that's OK in my opinion. But I like my home screens to look the same just so that I know my way around the most used functions of my phone - and I like it to look that way fairly quickly. After that I'll start messing with the 'new' stuff available to me.
What *I* do is this.
I use LauncherX, and have widgets set up that hold my most used applications (if you're not familiar with LauncherX I'm sure that sounds a little strange, so do a little research on the Market or AppBrain). I have three LX widgets on my home screen that hold my 15 most used apps, so I just have to place those three widgets to get my home screen looking 'right'. On a different screen I have 3 other LX widgets that hold my most used utility and root related apps, place those my phone is fully functional as far as I'm concerned (I only really use two home screens).
Before I flash a new ROM I use Titanium backup to back up all of my user apps (I don't get too concerned about system apps, I figure the new ROM will have different ones and I don't want to mix them).
After I flash the ROM the first thing I download from the market is Titanium Backup, and I use that to restore my user apps and all their settings. So not only do I get all my favorite apps back, but TB has 'remembered' my settings for each app and set them appropriately. LauncherX is one of my user apps, so all of my widgets are back, place the main 6 on my home screen and I'm ready to go. Once a new ROM has booted on my phone, it only takes me about 10 minutes to make it 'look' right. After that I may spend hours messing with settings, but that's a different story.