I think it depends on the colour of the lipstick. If you're using a nude lipstick, or a my-lips-but-better shade, you don't have to use a liner. But if you're using a red, purple, or dark lipstick, it's helpful to line the lips first, and it should be as close to the lipstick shade as possible (unless you want to deliberately change or shift the lipstick's undertone). Or there are a couple of companies who make "clear" lipliners (Urban Decay and, very soon, OCC) you can use. Another trick, particularly if you want a sharp lipline, is to go around the lip line with concealer.