Scope Creep - good or bad?

A question came up in a training session we were running today about scope creep and how you need to manage it. Essentially you want to avoid it as much as possible in order to limit your risk in liability and HSE for example, plus costs included for variations to a contract. However, the point was made that sometimes it's unavoidable and you just need to get the work done as it would cost time and money in delays to projects. So really it's "how much is too much?" and how do you control it?