I can tell you when NOT to fill-up, top off with water... when batteries are COLD.
Start trickle charging if plates are covered, you really want 15-25% of the 'reserve between add-fill limits' present before starting a high rate charge - but if you top off cold batteries, once everything warms up and expands, the chances are good you'll have electrolyte overflowing by the time the charge is finishing.
Spot check them again if you wind up cooking the batteries and they are actively bubbling - once you have them warm & 80 or 90% charged you can top them off, and when equalizing you need to monitor them again.
If you're in a place that is very humid you might not loose much, if relative humidity is very low they could get thirsty.
I think you need to worry about stratification only if you can't warm the batteries up - sitting on cold concrete and only having ability to do a low-rate charge comes to mind.