I used to agree with Mark, but now I'm not so sure. I've done one R134a job the right way [change all the seals, drier, expansion valve, evacuate the system, lube & recharge], one the sloppy lazy way [replace the drier, lube and recharge]. I can't tell the difference between the two cars A/C operation.
Since I have two or three spare A/C compressors, if one goes out I'll just swap another one in. If you don't have the spares and equipment and have to buy an expensive rebuilt compressor then I'd lean more heavily towards having a pro do it.
just my $.02