In short, while returns for the May to October period may on average be less, they are positive and if anything, the returns for this period have been increasing. The one notable exception being the incredible selloff in 2008. Taxes are a more consistently important issue to consider.
http://www.westonbeckett.com/posts/sell-in-may.html
http://www.westonbeckett.com/posts/sell-in-may-redux.html
In short, while returns for the May to October period may on average be less, they are positive and if anything, the returns for this period have been increasing. The one notable exception being the incredible selloff in 2008. Taxes are a more consistently important issue to consider.