If they use PCMiler it's generally going to be pretty accurate. If they use Rand McNally or something else, like the customer's word for how far it's gonna be, then it's a crapshoot.
Some companies use Household Good Miles, which is the shortest, worst way to go, and some use Practical Miles, which is, well the most practical way to go. Sometimes the difference between the two are huge. I have found, at least with Panther, that the loaded miles are usually within about 15 miles, over or under, the actual mileage. If it's more than 15 miles short, I might make a one-line comment on the QC, just to let them know I'm aware of it.
I use the 50 miles thing (40-50) for a complaint. If it's 50 or so short, I'll take a close look at the Garmin and calculate the route using all of the different vehicle types, and then I'll do the shortest possible route with MS Streets and Trips. If I can't find one at all that's that short, meaning the trip cannot physically be made in that amount of miles, I'll complain, because all they've done is take a customer's word as to how far the trip is, or it's been figured for routing from billing address and not shipping addresses.
Once with Con-Way, I once had a run from Natchez, Mississippi to Nashville and the mileage on the load offer was immediately noticeably low for a 530 mile trip. I ran it several times before responding to the offer. Turns out, it was calculated using the Natchez Trace (450 miles), which is 50 MPH the whole way, which is bad enough, but it's totally restricted to commercial vehicles.
Called them up and said I'd do it, for the 530 loaded miles, but not for 450. They said it paid 450. I said pay someone else.
They paid the 530. If there was another truck somewhere near Natchez, they'd have paid the other truck, I'm sure. But 80 miles is too much to eat on a loaded run, even in a cargo van.
If I had made the run and then realized that I was shorted on miles, I wouldn't have gotten any money for the extra miles, but they'd have gotten an ear full.