I edit a print magazine and this would be a value judgement based on the individual advertiser and the amount involved. I can absorb the occasional bad debt as long as I don't let things get out of hand.
Luckily in a niche area most prospective advertisers are either known to me or are known to people that I know. That sort of peer pressure makes a big difference.
A 30-day credit means a 30-day delay in starting their advertising. In almost all cases, we require pre-payment, at least initially. If it's a client we've been working with for a while, we may setup different terms.
Big-time ad agencies balk at prepayment, and may stretch your net 30 terms. Smaller agencies may tell you that they won't pay until they get paid by their client. Sometimes, even big, brand-name companies will milk their payables and pay their agencies and other vendors very slowly.