Please, not when you are actually at the wheel.
Let's say, write them onto the (available here) Mileage Log right after you finish the trip!
Is it worth doing?
Yes, it is. Say you drive a hundred miles a week for Charity.
That is $14 by the 2016 standard.
At the end of a 52 week year, it comes out to you saving $728.00 dollars.
That helps pay for gas at least.
Here is a clip from IRS 170 - year 2014
(i) Standard mileage rate for use of passenger automobile.
For purposes of computing the deduction under this section for use of a passenger automobile, the standard mileage rate shall be 14 cents per mile.
(j) Denial of deduction for certain travel expenses.
No deduction shall be allowed under this section for traveling expenses (including amounts expended for meals and lodging) while away from home, whether paid directly or by reimbursement, unless there is no significant element of personal pleasure, recreation, or vacation in such travel.
(k) Disallowance of deductions in certain cases
For disallowance of deductions for contributions to or for the use of communist controlled organizations, see section 11(a) of the Internal Security Act of 1950 50 U.S.C. 790.