I can only speak to my experience with local WM... but this is how it goes for me....
I ask the ladies who work there.. Will I get my negatives back ? (35mm or 645) and they say... NO ! Only a cd with scans...
I tell them I will take a chance..... and this is what happens... for going on 2 years now...
35mm c-41..... no negatives returned.. crappy prints & crappy cd scans
35mm e-6....... takes about 3 weeks and the slides come back exactly as expected albeit not much cheaper than other alternatives... but there is no postage..
120/220 c-41 I always get back both negatives and prints for what seems to be about 10 cents per exposure.... same slow turnaround... about 3 weeks... prints are crappy but I think of it as if I am paying for processing...
120/220 e-6 ...... transparencies come back uncut, sleeved, rolled around a little red cardboard tube piece (just like the ones I have sent off to Dwayne's) and the quality is great.... same slow turnaround about 3 weeks....
120/220 B&W ...... comes back with negatives and prints (which aren't bad) but it is rather costly imho... not worth it given a local "real" photography store still processes B&W 35/120 for $5 per roll processing...
I always cross out the generic instructions... Write SEND OUT across that area and below in the area of special instructions put something like... Kodak Portra 160 / 220 c-41 Thanks.... and they always come back just fine.. well one roll didn't make it back but they gave me almost $20 for the expired roll of Kodak Portra 220 they lost and apologized profusely.....
In the process Ive gotten to know "Shirley" the one lady who works in that department with a lick of sense.. and she finally asked one day... If you don't mind me asking... What kind of pictures do you have on those ROLL things they send back ? When I explained they were transparencies, like large slides she had a very puzzled look on her face and then asked... What do you do with those things ? She is with the program now and usually calls to tell me my film is back and ready for pickup.... Honestly the other 4 or 5 girls who work back there are rude, insolent and lazy.... but hey, where else can you get a 32 exp roll of 220 c-41 for $3.20 ?
No guarantee it will work at your local WM.. but this has been my experience... I say shoot a roll and give it a shot.... But don't drop off 35mm c41... you will be disappointed !