Depends. I've let people go ahead of me who only had a few items or if their kids were being fussy, but someone who just barges into the line because they think they're special would get an as polite as my mood would allow, "The end of the line is back there. Wait your turn."
I haven't had to chastise many people for line jumping, though. Maybe I look scary or something; I don't know, but only one or two people have tried it.