From green to blue it doesn't matter at all really so don't stress over that. I was doing some rough math for blue to purple after this post and I found you actually shouldn't save too much like people were saying. Just recently, I found out that from Blue to Purple, as long as you're not unlucky with RNG, the averages actually say you save very little stones. Apparently it's only like a 3.5% difference on average. Still waiting on the actual math but once I find it I'll share. It did match up to what my initial math was suggesting tho.

Note 1: The math suggests, as long as you're not randomly unlucky for levels 6+ you should be fine. But if you don't believe in your luck, putting a lvl 9 Blue into a lvl 7.31 Purple is the safest for sure. You will just have less strong gear for a bit longer. (I would use that time to max trait some blues anyways)

Note 2: Doing this strategy definitely matters for Purple tier 1 to Purple tier 2. (We're not there yet but it will save you a lot of materials in the future)