I ALWAYS give the customer the choice, there is no job that -requires- my shop to do it in, but the words I use, every time, are; "is it cost effective".
I have had trucks that they built a cooler around and that truck will only come out when they tear down the cooler wall. I hardly ever have suggested that 5 high full container handler be 'brought to the shop' for a brake job, but I always offer that to the customer who has only 1each 2k to 5k cushion.
There are good reasons to keep the road guys free and not tied up with 6 to 8 hour jobs. Cost per man hour is one. Best utilization of capital assets (the van) is another.
I always use the line that the owner [of the company I work for] says "loaner" is a cuss word, but we have good -rentals- available, and will set a 'max time' limit on how long our part of the job will take, but we sure don't want them to have a truck for free while they decide on added repairs.
All that said, I am who usually makes the call as to what to suggest/sell to the customer, and I have to admit I allow my own workload to fit into where I steer the conversation (if I am really swamped, I will try to send more to the shop).