A surveyor will (or should be) making a valuation on the basis of recent comparable sales. I would always try and have the house looking reasonably neat and tidy if I were having a stranger visit but I don't think there's any need to go over the top.
I don't think you need to tidy/prepare the way you would for a viewing. I mean if the house is a complete state (which I'm sure yours isn't) and obviously neglected than that could affect the value, but not if there are a few toys around and the taps are not actually shining with polish.
We had ours valued (we were considering remortgaging) a few months ago by a surveyor and he definitely came in and spent about 20 minutes here. I think drive-by valuations have gone the way of 120% mortgages, they are ultra-cautious these days.
We are in the same boat and I will give it a bit of a clean before they come round but only for my own pride. I don't like strangers/visitors to come to my house if it's a tip (which it usually is!) A quick hoover, wipe round the bathrooms and tidy up is all I'll be doing x