I should probably clarify...
A 2 deg. swing in temperature or a 15 sec. variance in time can cause a noticeable difference but not necessarily a detrimental difference. Sometimes, a desireable difference, even.
Yes, photographers often vary their developing times and/or temperatures to achieve a desired result.
My point was that, for your first couple-few rolls of film, do what the instructions say. When you get a feel for the process, then you can experiment with time, temperature or pH.
You have to learn to walk before you can run a marathon.