You can deduct the cost of anthing but that doesn't mean that you would be allowed the deduction if you were audited and of course it might be considered as tax fraud. Most IRS agents just want some form of proof that you have a legit deduction. If you get a young female agent they might give you a lot of trouble as they have a chip on their shoulder and are out to prove that you are a dishonest person. Not picking on females, its just my experiance over many years.

The requirement of recordkeeping and receipts depends upon the type of entity you are:

If you are self-employed sole proprietor the requirement is very lax and as I said before you generally just have to have records and receipts but you do not have to have formal bookkeeping. You of course should have, just not required.

If you are a corporation, you are required to maintain a formal bookkeeping record using formal accounting methods to properly reflect your income and expenses. Those records require receipts for most items, however, generally you do not have to have receipts for travel overnight meals and misc away travel expenses if less than $75. However, you do still have to document the business purpose, date, time, etc. of the travel.