Crabgrass is such a royal pita, for more than one reason:
1) The customer who only wants you to cut their grass doesn't care, so you're wasting your time, some of these lawns are nothing but crabgrass... Truth be known, it's the only thing that grows good in the heat thou.
2) The customers who do care usually have an irrigation system, which, using any weed killer you really need 48 hours without water! Spray this crabgrass and that evening or the next morning their sprinkler system washes it all away. Yeah, go ahead and ask them to turn it off (some do, some say they will but forget, etc), don't forget to check the weather forecast and calculate this into it as well, once again your efforts get wasted a fair bit.
3) Any weed killer needs to be sprayed 30 days apart from seeding, gee, that about rules out spring and fall, and if you spray it in the hottest part of summer you stand a chance of killing the rest of the lawn as well. Oh yeah, it really doesn't start growing until June-July.
4) Pre-emergent? One word:
LOL
You have to spray the WHOLE yard with pre-e, there's no way to predict where it will come up (or if it will), there's a bit of a COST associated with spraying an entire lawn, nevermind convincing a customer in spring that their yard needs it, when the evidence does not yet exist. Post-emergent is nice because spot treatments are cost-effective, but for this to work first the crabgrass has to exist.
Just a lovely mix, I know all about it.
Drive-75 is what I use, mix some and always carry some with you, then spot treat what you find, every visit if need be.