I like lifesize bears front high as on all fours or one front foot raised.Raising the front a little higher than the rear,gets the face closer to eye level and it emphasizes the animal's size better than all four on the ground.
On hind feet,upright is not my favorite as most of the view is the bear's belly hair which is the thinnest on the body.The best hair is on the neck,sides and back so again,I like the front high pose to show this.
Keep in mind,for mounting purposes,tanned hides have a shelf life.They will last a long time as a wall hanger but that doesn't mean they can be soaked up and mounted.Usually,if not stored,tanned in a freezer,they should be mounted within a year or two.Talk to your taxidermist about that.Humidity,sunlight exposure and type of tan can all affect this.
This is what I mean by front high.