I do. I'm guilty!
I have ran into a few patterns that the measurements were not correct so you have to add more after figuring out how much more you need.
I also do that because I live 40 miles from a decent fabric store and I really don't want to go back for it.
Also I like to get an extra quarter to 1 full yard for stash.
Isn't it wonderful how you can come up with so many projects to do while shopping then put your stash away along with your project idea? Mine usually stay stashed!