from IRS PUB 463 page 24
Recordkeeping
If you deduct travel, gift, or transportation expenses,
you must be able to prove (substantiate)
certain elements of expense. This chapter
discusses the records you need to keep to
prove these expenses.
You can’t deduct amounts that you approximate
or estimate
if you were to ignore this rule and enter an estimate, then you would have to answer that you don't have written records. this could be a red flag for the IRS and your audit potential would be increased. if audited the IRS could (and should) disallow all these expenses and then assert additional taxes, penalties, and interest.
