All of my flooded batteries also have the slits, and filling them up to the bottom of the slit does not seem to be an issue as long as 2 conditions are met:
1. As Woof pointed out, don't add water when they are discharged. I'm inclined based on a couple observations I've had with mine in the past however to even go as far as to agree with the concept of adding during equalization. Even if you have to initiate one just for the purpose of watering. Reason being is that this is ultimately where the highest water levels will ever be observed, and the bubbling will only add to the effective height. Once the cells reach a modest tempurature during EQ, fill em up. Another is that striation (tendency of water and acid to separate because the acid is heavier) will be reduced due to the bubbling stirring the electrolyte. This avoids uneven "wear" of the plates in the cells.
2. I tend to target just slightly below the slit for the simple fact that depending on the size of the battery and the container your water is in, it can be easy to get them too high, invariably leading to electrolyte loss. It will either come up out of the vents during a later charge, or when you go to correct your mistake on an immediate level by sucking some of it back out. There are 2 significant problems with this happening:
A. If you go the "let it overflow" route, those who have had it happen know that its not a one time event to clean up the mess before the
top of the battery also overflows and the acid goes elsewhere and does whatever its going to do when it gets there. It can take multiple neutralization/soakup sessions before it gets back down low enough that the overflowing stops.
B. You can't predict to any precision how much electrolyte will be lost based solely on how much extra water was put into the cell if the overflow method is allowed to take place, and if you go with the "suck it back out" method, you (ideally) need to overfill each cell by the same amount before you go to remove it. Cell imbalance will either start or get worse when the specific gravity gets out of whack from cell to cell, leading to premature failure of the battery as a whole when the weakest cell succumbs to the extra stress and abuse it will endure in the charge/discharge cycles to come.
All of that being said, I stay in the 1/8" range away from the bottom of the slit, particularly on the smaller batteries. If you're dealing with a forklift battery, gallon jugs filled with your distilled will obviously be less of an issue.
Steve