Yeah some of them can be really bad with handing out your info. The ones like huggies, pampers, aptimal etc have all been fairly good really, no spam mail here but I hear that Bounty and Emma's Diary especially are really bad for it.
I did give them my real details as I didnt realise what they were like before, regretting that now as apparently they bombard you with crap and telesales well beyond your due date.