I'll make 1 block when starting a project to make sure I know how to make it and it comes out the correct size. I like taking my project into EQ8, redraw it so I know how many blocks of each I'll need. Then I'll calculate how many blocks I'll need for the project, write down how many sections for each block, then calculate how many sections I can get out of a stripe of fabric, write down how many strips I'll need and so on. Seems to keep me from making too many blocks this way. May seem alot of work but when fabric is getting so expensive I like to save where I can. The project I'm working on now is a MSQC pattern, her mock cathedral windows which she only goes into making a wallhanging size. I needed to make a queen size so taking it into EQ8 let me know that my mental calculations were not correct and I needed to make a whole lot more cuts. Did make some extras from 1 fabric as I didn't need the whole strip but this way I could arrange them as I wanted with the extras. I'm so happy I took the project into EQ8 as I would have thought I had enough cut when I had only a little over half of what I needed.
As to extras leftover blocks, sometimes I'll add them to the backing to use them up as long as it doesn't take too many seams.