Actually if your heel lifts it will affect you no matter how good or bad you are at skiing.
If your heel lifts, when you lean forward, your heel will raise and your weight will be on your toes. This position is not good to control the skis, and can also lead to calf problems.
If your heel moves freely when your leaning forward you need it sorting out.
There are a few things that can be done, depending on why the heel is lifting, its not always that the boot is to big. If you have a very tight achiles tendon, your heel could still lift even in the tightest boot.
Overall if you have any movement other then moving your toes, take your boots to a good bootfitting shop and have them looked at.