I think it depends on the form of cheating. Cheating on anything you can do through learning - yeah, that's cheating yourself. But (in my high school at least) I've noticed that many homework assignments are pure busyword: there is no intellectual value to be gained through their completion, and all you are doing is wasting your time when in reality you could put it to a better use.
In those cases, I would not feel guilty in the least, and do not think I would be cheating myself either.
I would have said the exact same thing at the time. As it turns out, many of the "busywork" tasks are things that you'll want to be able to do quickly, easily, and accurately as you build on your skillset.
Repetition is how you optimize your brain. Quick and accurate execution of inner-loop code is critical. The more you can optimize, the more effective of a tool your brain will be.
If some work isn't worthwhile, then doing it poorly won't matter in the long run. On the other hand, if it is worthwhile work, then doing it poorly is bad for you.
It doesn't matter how much value I place on an assignment? Yes, to me, it does!
I do not understand who decides what 'matters' in this case. I see it as such: To me, a value-less assignment which takes up a disproportionate amount of time is wrong; thus, make it take less time. If it gives me no value in return, and I'm not hurting anything by cutting corners, then I most certainly will cut corners. Is there a better solution?
It's a trade-off. Cut corners, learn more on your own because you have time. Don't cut corners and be honest, learn less.
In those cases, I would not feel guilty in the least, and do not think I would be cheating myself either.