Depends on individual really, like what course you're doing, how much money your parents/loans/grants are giving you and how much you like to spend.
You can make it through without one as I have this year, just spending a tenner on the cheap nights out and walking back, but I miss being able to buy clothes and not having to worry about money on a night out, so I'm planning to get one next year. My course is only about 15 hours a week, so I can afford a few nights a week working I think, but I know engineering courses or anything like medicine demand your whole time really.
All down to personal preference though. If you know you want one, get one sorted, but you could just see how it goes for a few weeks, as you won't want to be working in freshers week! Only downside to this is lots of jobs will already be taken by other skint students.
Hope that provided a bit of insight, everyones different though