I'm going to be installing a sea strainer on my boat this winter, been seeing many pictures with valves installed on the sea strainer, is the reason for when you open the top to clean the water from the engine side doesn't back flow and over flow to sea strainer ? Which side of the strainer should I put the valve on ? Drive side or engine/pump side ? Or do I not even need one ?

If your running a bravo type drive I don't think their needed, if running transom mounted pickup then yes, if you do install one put it in between the pickup and the strainer,

Okay sounds good

Also wether running outdrive pickup or thru hull the valve is good for flushing. I added strainers to mine and have valves that will be turned off when running on hose. All the water fed to the strainer can be sucked up by the pump.

Valve right at the pickup is great if a hose breaks or falls off. Completely useless anywhere else if the same happens. I like valves on all through hulls below the water line, right at the fitting.