Starching always helps stabilize fabric for cutting. Don't be afraid to starch.
Personally, spray starch doesn't work for me; aside from overspraying, I always seem to manage to scorch it. When I starch fabric, I use diluted Sta-Flo liquid starch and saturate the fabric using a painter's brush, toss the fabric in the dryer, then iron with steam.
Actually, cutting the length of the fabric means cutting parallel to the selvedge lines (not perpendicular, which is the usual). The keys to cutting straight strips are: (1) if you fold more than once, make sure all fold lines are equidistant from each other, and (2) most importantly, make sure that your ruler is exactly perpendicular to the fold line(s). How to do the latter has been a topic of previous threads. I use a second ruler along the fold line and butt my cutting ruler against it to make sure I am cutting exactly 90-degrees from the fold line.