I guess I need to know the subtext of the question.
If the question is, "I have/will have an MS, will it help me gain admission?" the answer is almost certainly, yes, it will help you. If you're effective at presenting yourself, you can turn almost any life experience you've had into an asset, and an MS is a nice, concrete accomplishment.
If the question is, "I'm not sure whether I can get into an MSTP, should I get a MS first before I try?" then the answer is less clear. It would depend on the program itself, and it would depend on what other opportunities you had. If you had a really good research tech position, with a good mentor and a real research project of your own, that's probably better than a master's program (both in terms of your actual experience and in terms of strengthening your candidacy).
On the other hand, if you don't have such an experience, and you can find a really impressive program you can do well in, and have a solid research experience in, that might be a good way to go. It really depends on the master's program you'd consider, and the other opportunities that might be available to you.
Good luck.