This is a very wide topic, in essence I would summarize it this way. You have to balance security (risk of being infected) vs. perceived performance gain. Disabling either (read/write) will punch a whole in your line of defense. Everything should be scanned always, no exceptions. VSE provides you with the flexibilty of 3 scanning profiles (High/Low and Default) which can be used to adapt the product to your needs. For the low risk profile you may want to disable scan on write, I would always recommend to scan on read as a minimum.