In reverse order -
What I mean is to make the image, then (on another drive) verify that the image was made and made correctly. It really sucks to think you have been backing up the system all this time just to find out it didn't make the image correctly and/or the data is corrupted. That means you wasted all this time because you have to start over again anyway. The reason I say check it on another drive is two-fold. 1st - It guarantees that the image is good as otherwise you couldn't recreate the data on another drive. 2nd - If it is bad, you still have the original to make an image from.
As for the differential images, it is true, you can get some garbage in there over time. This is particularly true when you delete programs that don't totally remove themselves or you have your data in the same partition (some programs actually require this,
). How I handle that depends on how long it has been since my original image and how much my system and/or requirements have changed. If it has been a long time and/or I have many upgrades, etc... I just start over from scratch and essentially toss out my old images. I take a snapshot of the start menu using screenshot captor and print it out to ensure I remember all the programs I want to reinstall, then I wipe the machine and start over. If it hasn't had many changes or the image isn't so old, then I just recreate the base image. This is why -RW media is so nice. Reformat the media and make a new image
. Again, don't forget to test the image. Differential images aren't nearly as critical to check, but the base image absolutely is.