It really depends on the size and scale of your project, what your priorities are, and if you have any leeway with your budget.
Architects can sometimes be poor at sticking to a budget, and some aren't 100% confident costing up their designs. Hiring a cost surveyor can help you to make sure you stick to costs and can save you money by tweaking the architect's designs.
You might not need one, but you might still want one. The larger the build, the more important they are and the more likely they are to cover the cost of their own fee in cost-savings for you.
I was reluctant to hire one for an extension to my property but my architect recommended that I did. I ended up hiring someone from http://www.accelconsulting.co.uk/
a Sussex quantity surveys - and, although it increased the initial costs, I'm sure it saved me money in the long run.