If it is not causing a problem - like air in the returns, or cavitation, it is probably not a problem. I have a Pentair Intelliflow pump that is continuously adjustable. When I run it on a very low speed I typically have about 2" of air sitting in the top of the pump basket, and the pump runs fine. I can get rid of that by turning the pump up hight to fill the basket and then slowing it down, but it does not cause any problems in operation.