Rough mixes, data files, masters, mp3's, not even a couple of tracks you recorded out of a full album should ever be handed to the client without being payed. Even if the client is due back for more tracks or a mastering session, you could hand them over a rough mix and later a dispute between rates could arise and the client may decide there are no longer happy with the current situation and not come back; if you did not get paid for the work you handed over, you just lost money.
This is sad to say but even friends should be held to a certain professionalism. Chances are your friends won't skip out on you and not come back, but if you don't take some type of stand it's very easy for them to get the wrong impression and take advantage. Unpaid debt can build up and then get pushed to the side, then be forgotten as they are working on more debt. It's kind of like the old saying my Mom used to say, "Before it even starts, nip it in the bud."