our policy is that you are always reimbursed as though you had left from the office, no matter where you left from. the exception is going from one site straight to the other, in which case you don't bill a phantom trip to the office. so if i drive from my home to site A, then B, then back home, i bill from the office to A, to B, to the office. the other exception is for on-call trips, where you can bill all the miles, from your home. the idea there is you're not commuting; you're on the job from the second you receive your page, and any mileage is billable.
this arrangement always works in the favor of the employee. the worst case scenario is that the first site is directly on the opposite side of the office from your home, and then it is no different than commuting to the office and leaving from there. any other location allows you to take a more direct route, and actually drive less miles than you would have if you had commuted to the office, then to the site. also, some sites are on the way home, so you get to bill for the mileage back to the office, even though you're headed in the other direction (free miles home). of course, sometimes you actually need to go to the office first thing or last thing in the day, but even then you're just billing your actual mileage, and the commute is on you.
i get about $800/month in mileage reimbursement, and pocket about 2/3 of it after gas costs (though there are maintenance and depreciation factors to account for).