If it's there everytime you check, it's probably not a good sign. When the oil is churning in the engine with air, it's going to foam a little bit, however it shouldn't stay. It's like boiling water with the lid on. Water, compressed in there with some air, heats up, starts moving, you see foam on top..unavoidable.
Motor oil, as a viscous fluid (and the fact that it contains additives), tries to get the foam to dissipate quickly as foaming retards heat conductivity and promotes engine wear. Things such as water (or coolant, as above) encourage foaming so that may be all it is. If you see bubbles on the dipstick consistently
, I'd say change the oil first. Perhaps it just has some water, coolant, etc. in it.