One of the little rules I live by is "Monogram Everything!" But, sometimes the last name initial goes in the middle of the monogram, and sometimes it's at the end. How should you choose?
The guideline I use at Moore Stitching is that if the middle letter of the monogram is larger, then it should represent your last name. This is the case for "circle monograms" like you'll find on the Monogram Navajo Clutch.
On designs where all three letters are the same size, I recommend people put their three initials in order, with their last name last. A good example of this is the Tropical Summer Clutch. The letters there are all the same size, so consider putting your initials in their natural order.
Don't forget, this is your canvas, and your project. You can have the letters in any order you want. When you're filling out the monogram request field, please put the letters in the order you want them to appear.