See https://github.com/bitcoin/bitcoin/pull/4250 and https://github.com/bitcoin/bitcoin/pull/3959
A future release will automagically figure out the right fee (or will figure out it doesn't need a fee) to get into a block quickly (or slowly, if you want to pay lower fees and are willing to wait).
As for your original question: set the -paytxfee command-line option for current Core releases and you WILL always pay a fee.