I assume you're talking about something like the Sharkbite? It is critical that the cut be square to the pipe, all rough edges be removed, the end reamed back to proper shape (often a result of the tubing cutter's wheel bending the end in) and that you debur the end. If using a Sharkbite, they make a tool specifically for deburring the end. It puts a slight beveled edge on the pipe. All of this is to ensure that you do not damage the o-ring(s) that create the seal. There should be nothing on the pipe (no paint, drywall compound, glue, stickers, etc.), but it does not need to be cleaned and polished like when soldering on a connection. Depending on the grit, emery paper might make it harder for the o-ring to seal...they like smooth surfaces!