It depends really. If you and the label have an agreed release date putting a short clip up would be fine. A 2 minute clip, starting half way through the intro and ending half way through the break so people can't rip it and then mix it. There would be no point in uploading a clip of the full song, because if you have to pay for the tune the majority of people will just rip it off soundcloud/youtube or whatever.
really it depends how far along you are, the tune in my sig is the second i finished, I quite liked the idea but didnt think it was good enough to bother sending to labels so gave it away. I got a good response so i figured my style is going in the right direction and since have been working on tunes but not putting anything up. Its true that labels dont like to sign stuff people have heard, so for that reason I haven't uploaded anything else. Best way to get your tunes heard is to try and meet producers that are already somewhere in the scene. Best place to do it is at raves and over the internet. I dont know what its like in the rest of the UK/world but in brighton I always get the opportunity to chat to DJ's. Hell the other week I was jamming with Grafix. You've just gotta get networking, not what you know, its who you know.