Meditation and 'not trying to change anything'

I've been practicing mindfulness meditation for around three months now. I've heard many times that a key aim for meditation is to be with whatever thoughts and feelings are present, without trying to change anything. I understand to a large extent why this is important, as much upset is caused by suppressing and mentally running away from things that we are afraid of. However, in my practice I have found that if I consciously try to relax during a session (muscular tension is an issue with me) it can be a much more fulfilling, refreshing experience. Is this not an example of trying to change something, and therefore 'bad' practice?