I don't think it makes any real difference, since the charging rate is higher than the rate at which BT drains the battery.
I think what you're seeing is normal - if I understand correctly, the way these (and most modern rechargeable devices) work is that they fast charge to about 90%, then the charge rate drops until charged fully. After that, the device cycles between discharging/trickle charging to keep it at, or very close to, fully charged status until taken off the charger. It stands to reason therefore that sometimes you'll see slightly less than 100% charge level at the start of the day - but doesn't signify a problem.