Technically a track is perfectly in key at 0%, and +/-6% is exactly one whole key up or down. Therefore up to +/-3% it will stay in the same key as it was on 0% but any more than +/-3% you will be closer to the key it would be at +/-6% (to figure out what the new key of the track would be at +/-6% go up or down 7 places on a Camelot Wheel). This is because 3 is the halfway point between 0 and 6, however a track played around the +/-3% mark is actually lying kinda between the two and will sound quite off key, and should be avoided if possible. I tend to either stick below about a +/-2.1ish% change or go the whole hog and go up or down more than 4% and make the necessary calculations. Sometimes although a track might initially appear to be in a harmonic key, it might not work so well once you’ve got it in time if you have to speed it up or slow it down too much, in this instance it is often better to just to choose another track which will go better.
All this is just theory stuff though and in practice I reckon its fine to break any or all of these guidelines. If it feels good, do it!
*edit: you may be referring to how sometimes things can sound pretty ragged with master tempo switched on, particularly if the track is slowed down a lot. This is due to artifacts being added to the sound, which degrades it. IMO the better the production of the track, the more noticeable the effect of master tempo. Turn it off.
say I have 2 [A Minor] tracks. 1 of them is 128bpm and the other is 132bpm. The 128 track is playing, what if I pitch it up to 130 and mix the 2nd track (132bpm) in at 130bpm?