It's true. And "'Ask' comes before 'Assume'" could maybe do some of the work of "You know what happens when you assume: you make an ASS of YOU and ME." That saying puts the onus of the assumption on both the assumer and the assumee (is that a word? I don't know and I don't care! I created a slogan today!), when in my opinion the onus really should weigh most on the assumer. (I say that as someone who's sometimes assumed too much. I've been an ass before. I admit it.)
So. "'Ask' comes before 'Assume.'" Use it well.*
* Context! I was doing a paperwork thing, realized I thought I needed to do Action X with that paperwork but didn't know, so I asked and found I'd assumed wrong, so I did Action Z instead and realized the line that began this entry. You know how it began! It began "'Ask' comes before 'Assume'..."