Checking only things that you think have changed is a heck of a lot of coding for significantly less benefit. That extra coding introduces even more bugs and more maintenance time, but doesn't add any benefit. Anything I can catch by testing only things that have changed I can catch by testing everything.
Never assume that the world is a perfect place. If I can test everything every day with as much effort (or even less effort) than trying to be clever about it, I'll check everything everyday. Computers are designed to do mindless tasks over and over again.
Unless you are looking to waste time, once you accomplish the task, move on to the next thing on your To Do list. :)
I like Joel Spolsky's take on this sort of thing: Fixing bugs is only important when the value of having the bug fixed exceeds the cost of the fixing it.
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