Brine level goes down as pickles removed - top off or not?

Keeping vegetables submerged is important in a lacto-fermented product, because the fermentation is anaerobic (happens in the absence of oxygen). Keeping the product submerged during fermentation also makes it less likely that mold will grow on the product.

Refrigerator pickles are not generally fermented, and are just kept in a vinegar and seasoning base to flavor the product. Refrigeration also drastically slows any problem bacteria and such. Just avoid sticking your fingers in the jar to keep potential mold at bay, and given that you will likely consume in relatively short order, I would say there is no need to top off.