Why do you want to buy a TENS machine?
Is it because some health professional has used one on your shoulder before and you think it helped?
It really depends on your condition. There may be some use post surgery or in a case where you have to take painkiller in order to get through the day, however pain will remind you to look after your shoulder post aggrivation, not something that's entirely pointless.
What i have observed is many health professional who don't know what the are doing use these machines as the main stable of treatment, ignoring any poor biomechanics of the shoulders, getting the patients to return time after time while really the only thing settling down the symptoms is that fact that they haven't been to the gym for a month or two while paying someone for the benifit. And guess what the pain returns when you start lifting weights again...
The only other time I would see a use for it is in the population who are to scared to move their arm for fear of pain (creating the risk of frozen shoulder). This however is probably not the weightlifting population as they tend to be slightly tougher than this.
Some could argue that it is useful in the case of hypersensitive structures, this however would probably be over thinking the situation.
Remember the use of TENS is to trick the body into thinking it has no pathlogy. Not to stimulate recovery as some people claim. It's called the pain gait theory. Simply put if you bang your head and you rub it it feels better. Simply from the additional stimulation not from the fact you are fixing the bump on your head.
Just my take, mind you many health professionals have spent their entire careers using such machines.