@PostStat4ever and @IBrakeForStamps --
DMM Art. 604 Sect. 3.5 says:
"Stamp Collectors: Subject to USPS policy in 608.4.0, precanceled postage may be bought for philatelic purposes as well as postage payment. A stamp collector may mail matter bearing precanceled postage if the collector has a permit to use precanceled postage at the Post Office where the mail is presented. Creating philatelic oddities or precanceling issues or denominations that would not otherwise see legitimate mail use is not permitted. Mail orders must include a stamped, self-addressed envelope." (Read here:
http://pe.usps.com/text/DMM300/604.htm#1080435)This is to show that MPP holders are permitted to use precancelled postage. I have.
From my Boerne, Tex. MPP:

From my San Antonio, Tex. MPP:


You'll notice that the above images show precancelled stamps on domestic postcards and 1 oz. First Class Mail, as well as non-domestic.
The key to preventing these from getting overpostmarked -- sprayed -- is to have the postal clerk separate your mail and note that it's already been cancelled. Seeing that I do not expect them to write a note on my items, I include a printout when I hand them my postmarked mail. This is important so that the whole chain of workers -- and I have no idea how many handle my mail beyond the counter -- understand what they are dealing with.

The sheet works well! In fact, when I mailed some postcards from San Diego last month I wrote out such a sheet by hand, had the clerk hand-cancel my postcards with the office's CDS, and asked her to place my postcards in their own bin along with the note. She went out of her way to do it for my sake, and, what do you know!, the hand cancelled items made it to their destinations without a spray cancel. This, of course, did rely on a willing postal worker which are not at all offices. :)
Back to mail bearing precanceled stamps -- I use a hand stamp that cites the DMM to make it clear that I use the stamps in accord with official policy. This eliminates questions, especially since very few believe it is possible for Joe Q to use precancelled postage.
Here is an example:

And, to end this, here is an example of a processed cover -- see the bar code at the bottom -- that was not machine cancelled and not even marked "non-machinable" (which comes at a cost). This is also domestic, within the USPS' territory. It's my first day of issue on my Kyle, Tx. MPP #1 back in spring.

Handcancelled mail is fun and has its rewards -- and international philatelists I know think it is amazing that we are able to cancel our own mail in the United States. A penpal of mine in Japan said she is quite envious of our ability to hand cancel our own mail for philatelic purposes.
Cheers!

Kyle