I don't really take many baths any more, because my hair is long enough that it's a PITA to wash it, and I'd rather relax with a blanket and a heater vent than hot water, but when we were kids we always had baths. Bath got cleaned once a week, and other than that it was just water in, bathe, water out again, nothing fancy. There was never much of a gunk build up of any sort, really. (Not like now, when the bath gets covered in layers of dust, and needs a good clean before it gets used because dustbunnies are not relaxing.) If there was bubble bath residue, enough water got splashed on it that it went down the drain too, and that was pretty much it. I dunno if that helps, but yeah, I wouldn't worry about it too much.