Some tub makers mention not adding soft water in the owner's booklet, other put it right in the warranty.
If you are going to add part soft water and part hard, keep in mind that the odd shape of most tubs puts the 'halfway' point at about ten inches from the top of the tub.
I live in Southern California, and some of my customers need to fill the tub with hard water, except for the last three inches. That comes out to roughly 20-25% for the HotSpring and Tiger River that I sell. It works fine that way.
If you add all soft water and then bring the hardness up with the addition of hardness increaser, you can be adding to the TDS level, and you also may have more salt than usual. It would be a good idea to test for these two items if you use the "add soft and raise it" technique.
One last item: some folks simply add all hard water and then put in soft as they need to refill the tub.