I don't have "paid" writing from "non-paid" writing at this point. My writing is primarily fiction but I occasionally get the urge to write an article and submit it somewhere.
I write every night after my youngest child has gone to bed, and try to put in at least a few hours. Very rarely I just don't feel like it and have a night just veging out in front of the idiot box. Like I said though - rare.
If things are particularly slow at work - I sometimes find time to do some writing there and strangely I seem to get some interesting stuff done at those times. If I could only find the winning lotto numbers I could retire and write during the day - all the time 
If you're getting your "paid" writing done by the due deadlines, I don't think it's a big thing if your fiction slips in occasionally, although I wouldn't make it a big habit.
As long as you're writing something, preferably every day, then I don't think you have an issue.