Since your running an 18" Id recommend 215/35/18. Yes it will be a slightly stiffer ride, but you'll be fine as long as you avoid the big poth holes! I wouldnt recommend taking much advice from your average local shop. As woodbridge alignment told me my car was dangerous to drive because my suspension is too stiff.....
To help tuck your wheels in (rears) id recommend getting adjustible rear control arms. The prices vary by brand. start at about 180 for spc's to about 450 for UUC's.