That's an overly simplistic, and rather naive view of how the market works.
There's no guarantee prices will go up, if we knew with that kind of certainty wouldn't we all be swimming in money?
The theory is based on the principle that people sell before the end of June to realose any tax losses, and will do so at any price driving prices down.
Personally IMO, in all my time watching markets, I don't think it's a factor at all.
