Actually, extra punctuation is a bit like swearing: if you do it too much people will get offended, and 'doing it more adds more emphasis' is only true up to a point, after which you just seem like a bit of a moron.
e.g.:
'No!' is the equivalent of 'No!'
'No!!' is the equivalent of 'No, bugger it, no!'
'No!!!' is, in the rare situations in which it's justified (bearing in mind you're talking to strangers), the equivalent of 'Fuck no!'
'No!!!!' is the equivalent of 'fuckety fuckety shit-wank no, yah cunt!'. In other words, it doesn't make you seem extra-emphatic, it just makes you seem silly.