This is very interesting question. I will keep the answer of this question very simple. First of all there is no scientific research or white paper I can backup my results with. Answer contains part simple observation and part experience.
There is no need to reboot SQL Server. Once it is on it is ON!
However, I have heard that frequent reboot improves performance.
In my company our network administration department has policy to reboot all the servers every 15 days. We reboot all the servers at every 15 days. Regarding performance improvement, our servers are always up and running as great as they can.
Reference : Pinal Dave (http://blog.SQLAuthority.com)