I bend quite a few of the directions on products, but I like to think I have a bit of an understanding about how they work when I do so. I'm probably incorrect often enough, that understanding is a work in progress. And its taken me a long time, and countless hours of reading and tinkering to get whatever "understanding" I have about things.
Certainly more than your average diy'er will put into trying to understand things.
So suggesting going against manufacture directions, mixing products, etc amongst other pro's is probably ok. They should all understand what is going on.
Its not really ok for diy'ers because they don't have that same understanding, and will just take advice literally. That can be bad if you think about some of the misinformation out there.
I've heard some good ones being preached at our local building supply.
"The showers leak because the grout lines are to tight for grout to get in and seal it up"
"just put cement board up and caulk the joints and its waterproof"
"It won't leak if you put sealer on it"
and my favorite...
"showers just get moldy grout lines, its the way it is. That's why you use a dark grout"
I'm sure everyone has heard some "gooders"
(That might make an interesting thread!

)
I have used mastic before also, and would again if need be. But care should be taken when giving advice to diy'ers.
I once had to help someone out once because their floor all fell apart. The store had sent them home to install cheapo 12"x12" ceramic tile on a concrete slab, with mastic, and a 1/8" v-notch trowel...