Auto and home loans are generally not payable directly to the lender by a credit card. There are services that will make these payments but will charge you a fee for doing so. And even those lenders who will take the payments by credit card will charge you the credit card fee (roughly 3%) for the privilege. It's generally not worth the expense to do this--as you can figure out, the more you pay these fees, the less money you'll have for other credit card bonus spending requirements.

Look at your monthly after-tax income. Figure most expenses after auto and home loans and your savings can be run through a credit card. Utilities tend to be split, with power companies in many areas again imposing a fee on payments by credit card while communications companies (cell and home phones, internet services, cable services) do not. Many insurance payments may be made by credit cards without incurring a fee. Do a budget that you can run through a credit card, and you'll then know how much you can afford monthly in meeting bonus spending requirements.

As you've also already figured out, it's not a bad thing to be the banker for friends who are making purchases--just keep in mind the two dangers here. First, your friends have to be reliable--cash (or check deposited into your account) up front. Second, you've got to be disciplined with the cash (or your suddenly larger account balance)--spend this money on something other than paying off your card, and you'll start incurring big interest charges, fast.