It's worst from 1 to 20...after 20 it thins out a bit, and after 30 they all either know you're a multiboxer or just don't care. It's mostly the kids with their twinks and low attention spans that can't lvl past 30 that make a big deal over it. Though there's an element of luck too in how well your server's pop knows about multiboxers, ranging from well-versed Magtheridon, home of the Zins & Zerg, down to some unknown servers who've never seen a multiboxer before and missed the fun threads we have onthe official forums ;p

But yea, keep on truckin, it'll slow down in later lvls; however some 70s(coughEbaycough) will still harass you probably or follow ya, just remember it'll be different per person, some might follow and help you, others might be interested in MBing, and a few're just curious or watching out of boredom