Probably not the answer you were looking for but .... "It depends".
Seriously, there are times when I do it intentionally and there are times when i intentionally DON'T do it. Depends what feel the song needs. I like doing it when the song needs to be slower and/or heavier. I also do it when it needs to be fast and have a feeling like it's being pushed along.
On the other hands, there are times when the guitar is just chugging out power chords and I'll play something melodic under it. Or even if the guitars are playing their own melody it can be interesting to counter play a counter melody.
But to sum it up ..... variety!!! That's one of the many things that makes music interesting. Throw it in and see what works and what doesn't. Not only will your songs improve but you will over time, too.
