Do I need a printed receipt to go with a cancelled check for our income tax (itemized deductions)?

If you have an invoice for the item and a cancelled check indicating that the invoice was paid, the IRS will usually accept that at face value. If you don't have an invoice a receipt will be required.

You need to substantiate what you paid for and the fact that you actually paid it. In and of itself a cancelled check only proves payment, not what was paid for. A receipt marked "Paid" is the gold standard but an invoice and a cancelled check is a very strong second -- good enough that I've never seen the IRS reject it unless the validity of the invoice was called into question.

The same rules apply for credit card statements. Coupled with an invoice it will nearly always suffice but on its own a credit card statement seldom includes enough information to substantiate what was paid for, only that payment was made.

The IRS no longer accepts cancelled checks as the only proof of payment. You must have a receipt to go along with it.

Yes. You need proof of what the money was for. A check isn't going to say who the services were for, what the services were, etc.

He could take me to his doctor and pay for the visit with a check. Perfectly legal, but not deductible since I am not him, his spouse or a dependent.

normally tax preparers have a proforma they ask you to provide in which you list all your income and the expenses that you are claiming including the Sch A that you are providing information, in the envelope they provide you will be all the receipts you have to prove your deductions, they are not going to take the time to compare all, they are depending on you to provide the information of your own return, since it is YOUR own return, it is not theirs and it is you that is responsible to provide the right information

The check is the receipt. But an actual invoice showing what the payment was for sure helps. For example, if you're writing a check to Farmer's, the invoice will show that you're paying your homeowner's insurance, not just your car insurance.

It depends on the deduction.