I personally think that the placement of the tip doesn't matter too much. The upward force supporting/pushing on the palate should originate at the very root of the tongue. This is the area under the jaw, near the throat and the hyoid bone. The upward force generated by the root should be transmitted to the palate through the visible tongue. So I don't think it truly matters if the tongue slides back or forward a little bit since it is merely transmitting forces to the palate.

I've recently realized that the placement of the tip of the tongue is actually crucial. Maybe at an intial stage of mewing, when one is very recessed and can't fit their whole tongue on their pallet, it doesn't make as much a difference, but at the later stages it definitely matters, I might draw up a diagram later to illustrate my point. 

Anyway, point is, the tip definitely should be pushing at forward/vertical vector right above the front incisors.

I put more importance on palatoglossus activation since that has the most chance to change the pitch of the maxilla. As someone who has a Class 3 AND retroclined incisors, I focus on posterior engagement of the tongue and then on using my tongue to put pressure on the whole palate + the incisors and canines. This would cause the maxilla to widen at all points, preventing bimax protrusion.