Well I know that whenever I get my hands on Dreams I will not stress about thermo use and stretching in/out of create mode. I'll just use the optimization tool whenever the thermo is full. Perhaps using clone tool is the only thing to keep in mind.
Unless someone in the beta says otherwise, stretching in sculpt mode then reducing detail with the optimization tool or stretching outside of sculpt mode directly should end up being more or less the same.
I think a good analogy is say you create a square in Dreams, see that as creating a 32x32 image. Then you stretch it in create mode this is like making it a 512x512 image which takes up a lot more memory/thermo space. Whereas if you stretch it outside create mode this is like keeping your 32x32 image but just rendering an up-scaled version. The upscaled 32x32 image is a lot less detailed than the native 512x512 image but since it is just a square you don't see a big difference between the upscaled image and the hi-res image. The hi-res image is overly detailed for what it is and needs to be compressed. This is the optimization tool that shows you which object consumes most of the memory. It is up to you to decide whether it is justified or not.
TLDR; Don't worry about stretching. Use the optimization tool when your thermo is full.
It really depends on the subject matter. If you're doing a soft, dreamy look then using the optimization tool is hardly noticeable. If you're going for very precise shapes with sharp, well-defined edges, you have to be careful about overdoing it and with how you build things.
That being said, so far I haven't hit any big thermo walls, so as John said it's a bit more guidance than hard limits. But I do wish the UI made those decisions more obvious to the user.