the general 'rule of thumb' for pin basting is every hand width...so each place you put your fist should touch a pin top, bottom, side & side- or about every 4 inches...they believe everyone's fist is about 4" across :)
as for your scraps...it depends on what kind of a quilter you are...i applique alot so i can use some pretty small pieces of fabric at times...i have a friend who tosses everything left from a project...(finally convinced her to not throw it away, she donates it to the senior center now) all i ever actually throw away is slivers...little bits trimmed from edges of blocks. i keep a couple baskets under the cutting table. one for strips (for rug hooking, wrapping clothesline, string quilts) one for small pieces; (cut off triangle corners, anything too small to fold) when i have been working on alot of different projects and the scraps are really getting abundant i look to see if anyone has put out a request for some, if so i box them up and mail them out...if no one is looking for what i have at that time i take them to the local pre-school or elementary school art department. the teachers really appreciate the donations and the kids are amazing with their creativity!