Showering at her place before the session may have a dual function.
First, as many have noted, it puts her at ease and will make the session work better for both parties.
Second, as has already been mentioned (viz. CFNM), it may serve as a condition which makes it possible for her to actively take the initiative with the session (which is probably in the interest of most people, with their list of services which they wish to be actualised).
If you think maybe you would still like to be clothed at the beginning of the session (after having freshly showered), you may be better off working out beforehand that you want a 'girlfriend experience', because otherwise it may just be assumed that you want to get 'straight down to business'.
From my experience, appearing with clothes on again after the shower brings about confusion (which again may be 'headed off at the pass' by sorting out what kind of session you are really looking for).
Perhaps there is a learnt behavioural code for an 'in call' which one should pick up and go along with. Like other formalised social conventions which one tacitly performs.