people use different methods for making their log-cabin blocks- i generally cut my strips the width i need- then may cut them into a (manageable) length---not leave them 42" long- but not try to cut them to size either- i sew on a log- cut it even - add the next log, cut it even- go around my cabin- i use a foundation to keep everything stable (generally i draw my blocks on lightweight muslin for a foundation to work on-that way i don't have to remove anything) keeps everything from getting wonky- it works for me- i've not had any luck cutting my logs to size in advance- they always wind up being too short, or too long somewhere along the way-
check out the Judy Martin Log cabin book- she has a special ruler that i have heard great things about- and things (in theory) come out very accurate. but (in my opinion) if you have already found that your logs wind up being short---leave them a little long & trim them to size as you add them---just me...