Absolutely, positively makes a big difference in imaging and sound staging in my experience. I'm assuming you are comparing to just putting them on a shelf - you didn't really specify what the 'difference' was in comparison to (on the floor, against the wall, on a shelf, on a table). Anyway, I can say with great confidence that a pair of monitors will do better on stands than any of those options, and will also give you the flexibility of being able to move them around to their best advantage in the space you are using them in. If you can find a pair of Osiris stands (no longer made, but excellent, fillable stands...very heavy even unfilled) on the used market they've worked great for me. They look good too. Usually around $250-300.
Marco
Marco