Pat of course you can combine these two techniques. Usually when I pre sash, I add the sashing to the right side and the bottom of each block. If I want to pre-sash the whole quilt, then I add the left sash to the first left block also. All the top row blocks also get the top sash.
If I want long strips on the outer edges of the center, I do this instead:
Start with your first top left block and presash the right side and bottom, continue that row the same way, but on the far right last block on the first row, do not do the right side sash, only the bottom sash. Repeat for all rows. But on the last row, do not sash the bottoms at all. Now when you sew these rows together, they will be all pre-sashed in the centers, but not on the top, left side, right side or bottom. Then you can add those last 4 as long strips.
Clear as mud??