Cruising RPM. can it be to low ?

Its all about efficiency at the throttle position you are at. We dyno engines at full throttle, but for the most part, mods to generate more overall power, mean less efficiency at less throttle openings. Some designs, and some management systems can help ease the compromise between the power vs part throttle efficiency, but it's all dependent on what you have. IMO, as long as you are past your full mechanical advance, and run a vacuum advance on ported source, you are fine. But not all combo's have that, or will run well at low rpm with it.