Actually, it kind of is more prominent now.  It used to be the case that a sheared Box would use this method (incorrectly), since the SBTransform layer bypassed the overloaded SBBox::fillXGrid function, and instead called a bunch of xValues.  Now, with the fillXValue functions, it does get back to the SBBox overload.  When it does, it hits an exception saying that we haven't implemented SBBox::fillXValue for non-square grids, which invites people to write that code if they actually need it.  So that pretty much is the more prominent warning you are suggesting.