I've actually done some resecent research into the tax status of Scouting units. You can only claim a tax deduction for unreimbursed out-of pocket expenses, such as unreimbursed fuel costs, if the unit's chartering organization is a qualified charitable organization. I know that teh vast majority of the units qualify as being tax exempt, as they are chartered by either a church or civic organization (such as the American Legion). Even most "Parents of" or "Friends of" organizations should be able to qualify as such. But I have heard of a few "odd" chartering organizations that do not qualify as qualified charitable organizations, such as units chartered by a local business. Technically speaking, expenses incurred on behalf of those units are not tax deductable.
You should know that although the BSA is a qualified charitable organization, under IRS regulations BSA has the ability to control who is able to use that tax status and who is not. Although BSA has passed that tax-exempt status on to your Councel (including FOS), they have elected NOT to pass that tax status down to the unit level. Becuase of that your Scouting Unit derives its tax status from its chartering organization.
More infomration is available here:
http://www.gpc-bsa.org/finance/unittaxexemptstatus.asp