In those cases, I would not feel guilty in the least, and do not think I would be cheating myself either.
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.
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.