I buy them when I have some extra USD so mostly at the start of the month, I also buy as much as I can when I see big dips with nothing supporting the downward spiral of the currency, I exchange some for alt-coins as well when a good coin is getting more popular etc...
That's the right thing to do, if we have faith in potential of bitcoin and if we can manage to hold if for long term then we can afford to buy at any price, as price will surely rise in long term.