I can surely understand your concern and I think that the best thing for you to do is to be up front when asked about the situation.  Explain exactly as you have here as to why you are not living together at this time in the same matter of fact manner and that you are with each other as often as time and finances permit.  The issue is always if the marriage is a true union and not a sham for purposes of obtaining citizenship.  That is why there are in depth investigations in to relationships and marriages by INS.  If you are still leery speak with an immigration attorney.  Good luck to you.