If you do then you may be limiting your composition. I don't always know which format is going to make the most pleasing photo until I get home and see it full size on a monitor. By then it is too late as I can never re-create that scene again. So I usually take one shot in landscape format and a similar shot in portrait format to hedge my bets. This is especially important in landscape photography.
Here are two examples to illustrate what I mean. Both taken of the same scene at Dawlish Warren, Devon.
1. LANDSCAPE FORMAT
2. PORTRAIT FORMAT
In this instance, I think I prefer the portrait format because it is less cluttered and better structured. If I hadn't taken it, I would have had difficulty cropping the landscape shot to get a similar photo.