Are bicycle registration websites any use for recovering stolen bikes?
The two questions that I think matter are:


which sites do your local bikes shops check all the bikes that come in against?
which sites do your local police check bikes they receive against?


In my experience bike shops very rarely check bikes at all, largely because so few are registered (and a relatively low proportion of serviced bikes are stolen). When we have checked it's been because of something like filed-off serial numbers or a DIY repaint job, and we've not ever had any joy doing so. Even ringing the police in one case didn help - the bike hadn't been reported as stolen.

As far as the police go, I've been told by one officer that at least in Australia the best you can hope for is them typing the serial number(s) into google. 

You're better off keeping a record of the serial number and reporting your bike stolen with those details, because the police do actually check serial numbers (it's built into the computer systems).

One thing that also helps is carving your ID number (national ID, driver's licence, whatever is common in your country) into the frame or something obvious like the outside face of the cranks. That makes it irritating to remove and again, easy for the police to identify but not easy for anyone else. If you have access to an engraving tool itś not hard to do this (practice on something first!), but of course it also makes your bike hard to sell.