When I was an apprentice, I operated all of my birth work like a business. Some parts of the business made money, some lost money. Doula work was profitable, midwifery work, where I was paid between $50 and $200 per birth, was not. Doing it this way made it so that I could deduct all of my mileage from my taxes as well as supplies and equipment, workshops, magazine subscriptions, and memberships. By paying me a general fee per birth, my preceptors made it possible legally for me to call my work for them a job and deduct all of that stuff. I ended up with a business loss and it helped on my family taxes (I think you can declare a loss only 2 of every 3 years or the IRS gets upset, but it was legit that year). If I had just been reimbursed for mileage, even though it might have been similar to the $50 payment, then I'm not sure I would have.
I try to drive my apprentices around as much as possible. Some of them don't live near me, so it doesn't work all that well, but I do try to limit their fuel expenses. I have specifically offered mileage reimbursement once when I had an apprentice run a lengthy errand. Otherwise, I will work up to paying my apprentices the same kind of fees that I was paid.