a note on margins and paper sizes
You can skip this part unless you know you have particular print dimensions in mind. It's a lot of math.
Traditional paper sizes are usually in a 4x5 or 3x4 shape, much like a standard television or computer monitor. Some images have the same aspect ratio, either by cropping or due to the camera used to take them. Traditional paper sizes include 5x7, 8x10, 16x20, and 20x24.
Most images have a full frame ratio of 2x3, which is a bit more rectangular, and more like a widescreen TV or monitor, or a movie screen. (All three have different ratios, but are all more rectangular than square.) Full frame paper sizes include 8x12, 10x15, 11x16, 12x18, 16x24, and 20x30.
And of course, a square image is... square. Also, some images, by the nature of the image details, crop easily to any number of paper sizes and aspect ratios. Others need the exact shape they have.
Most standard matte openings are slightly smaller than the listed image size, and thus the matte covers up some of the image.
So ... I typically print images with a border. The border on the top, left and right is often about 1/4 inch. This keeps image out from under the matte. The bottom margin will depend on the ratio of the image as compared to the ratio of the paper. An image with the same ratio as the paper will have 1/4 of an inch on the bottom as well. A full frame image (2x3) on traditional paper sizes (3x4) will have a larger margin. For example, a panoramic image on 11x14 paper might be 13.5 inches wide by 7 inches tall. I can print all of that margin at the bottom, or I can split it evenly. I can use larger margins - a sort of self matting effect. An example of that might be a 16x20 print of a full frame image, with about 1 inch margins on the top, left and right, and a 3 inch margin on the bottom. The margins are usually white, but can also be black or whatever other color you'd like.
And naturally, if you're custom matting (or custom matting and framing both) then you can do just about any size print, any size paper, any size margin.
So what does all this mean? It means options. For my own images at home, I've got a mix - some printed full to the edges, for frames without mattes, and some are the self-matted style, with larger borders serving as a matte for the image - there's no one answer. While printing to order based on web previews means you can't just pluck a framed print off the wall and take it home as is, it also means you can request what you like.
If you don't have a preference, don't worry about any of this. I'll choose what looks best for the image. If you do have a preference, I can do whatever you like. Feel free to ask about the best options for certain images.