We do them ourselves - I didn't like paying and waiting so why not? Site survey: http://www.tamos.com/products/wifi-site-survey/. Seems feature-rich enough, works well (i.e. good predictive results) and costs a lot less. Regular updates including Extreme APs. I'm not a huge fan of its reporting features but again, the cost makes it ok.
You should do predictive and post active if you can. If you don't have time but you have the experience, guess with some spot-checking. If you aren't comfortable with that, get a partner to do the surveys(check credentials/referrals!).
Yearly would be nice but I don't think the ROI (timewise) would be worth it. This stuff runs rock-solid and with proper setup I have little issue. The very few problems it would unearth wouldn't justify the time it took to walk the entire campus.
Every site should have a pre- and post survey done. One to plan out the install and one to confirm your deployment covers the business needs. The method of performing a survey can vary greatly depending on the tools you have or don't have. Sometimes a 3rd party can be contracted to perform this for you or you can purchase the tools to have on hand.
Personally, I use the NetScout Airmagnet Survey Pro suite. Works well and easy to use. There is also Ekahau similar to Airmagnet. Both do about the same. There are others, but these are the top 2.
You should do a yearly audit to make sure your APs are still where they are supposed to be and that they are functioning as they should providing wireless coverage.
I currently have responsibility for over 900 locations. Our internal team is a staff of 3 with 30 more out in the field. We cannot be everywhere. We trained the 30 on how to do the predictive based on what they know about their site\location. Like some of the other comments, not enough manpower to perform actual post surveys. We do rely upon the Extreme WiNG controllers and management to keep us posted on changes. Static versus smart-rf. Hmmm that is another discussion.
Regardless, surveys are the most important part of the wireless environment. You gotta have them and keep them current.