Quite often mortises are cut deep enough in small legs that they overlap. This means that the tenons are going to bump into one another when the piece is assembled. If you don’t bevel, or miter, the ends of the tenons, you’ll end up with open joints – that’s not a good thing.
I usually get asked a couple of questions revolving around mitering the tenons, “Should the miters touch?” and, “How do you cut the miters?”
To answer the questions in order, no the mitered tenons shouldn’t touch inside the joints. If they do, you end up stressing the joint as things begin to shrink, so leave a gap.
And if you’re leaving a gap, the answer to the second question is, any way you can. I’ve seen lots of people set up complicated rigs on their table saws or miter saws, but the truth is, if you aren’t going to make perfect miters then it doesn’t matter how you cut them. Most of the time I rough them off on the bandsaw. If that’s not accurate enough to satisfy your inner engineer, I suggest marking the 45° angles on the tenons and building a miter sled for your bandsaw. But make sure you figure in the gap.