I’m getting the last few items in the BOM, and have a question.
The screws for mounting the stepper are M5s of three different lengths. Aside from aesthetics, is there any reason I couldn’t just use M5x20mm screws rather than all the individual sizes?
You might have interference issues with the X motor. If you use 20mm on the y motors you will have to run them the opposite direction or they will hit your idler pulleys and you may not be able to get a nut on them this way. IMHO it’s best to get the correct size screws and save yourself the headache.
If you see a need to use a different length of bolt than supplied, check at Lowe’s or another hardware store. I was surprised to find a good assortment of metric bolts and nuts at our local Lowe’s when I wanted to add nylock nuts to some locations on my assembly.
Thanks guys. That’s exactly what I needed to know. Looks like I’ll be getting the proper sizes. 
If there were no clearance issues I was just going to get a 100 pack of 20mm screws.