I've only done it when cold and always in between the min and max lines.
Even that can readily be too zealous a topping up strategy, I suggest you refrain from going anything over mid-way "full".
The only immediate issue with the fluid being too low is the fluid can't be circulated by the pump, so whilst you have hot water you will not have space heating available, till it is topped up just enough for the pump to circulate the fluid.
Edit: two things in particular related to the fluid loss issue happen even when the system is not over filled and operating "normally".
Obviously the fluid heats up and with this the air in the header above the fluid. This air because of being on top of hot fluid and itself hot takes up much greater humidity, that is the water evaporates out of the fluid.
Both that air expands and so does the fluid, therefore some of that humid air together with the water it has picked up gets displaced out of the Adle vent. [unlike most cars that use a sealed system].
Then after use or lower heating demands things cool and the fluid shrinks, thus drawing local probably low humidity air into the header.
This endless cycling therefore progressively looses a trace of the fluid water which over a long period can be noticed. All we need to do in this case, which is not a leak, is top up with pure water, as the antifreeze and additives themselves are not lost. Topping up with mixture in the small quantities though will not have any real adverse effects.