When I considerd this question for my own business I thought about what areas I wanted to be working in & just who could afford what I was going to charge. The area I live in isn't flooded with people who will pay for quality but more often they will look for the cheapest set of nails they can get. The areas I wanted to target were a bit more far afield so after working out just how much it costs me in petrol for every mile I drive (turned out to be 9p, that's on in a Rover 414 with a 1.4L engine, many on this site I've noticed have smaller cars so get more mpg) I set my limit as 20 miles. If a client wants me to travel further I have a travel surcharge stated in my terms & conditions of 50p per mile over the 20. First client I ever got was 30 miles away but was more than happy to pay the £5 extra as a friend had "sold" my services to her so well.
Suppose a lot of it boils down to what each person feels is an acceptable distance to drive, as in my past employments I have often driven huge distances to work I maybe think less of the driving than others do.
Plus working more out of the area I live in I never have to worry about a client spotting me out & about on a bad hair day or down B&Q in paint spattered overalls & nails looking battered when I'm on a DIY stint :lol: