I hate these sorts of decisions! As you are finding out, there are many different options. Not only to sash or not, but how wide, cornerstones or not, and whether to disappear into background or make a distinct grid among others. You have lovely blocks and it will be wonderful however you decide for this time, and there can always be a next time if you want.
If you go with sashing (especially if you are using cornerstones), many of us find that it goes on better as an L around the blocks than trying to connect long strips of blocks/sashing together. You also need to finish one of the sides and the bottom, again, it's easier done framing the blocks than with what I call long unwieldy strips.
There are other techniques to help when you are using a long solid piece of fabric as sashing. Some as simple as marking the center and or quarter lines of both pieces before joining, or the advantages of "cut to fit" as opposed to "fit to cut".
I've attached a sketch of an exchange 9-patch block showing the sashing already put on.