Good security dictates that you choose a trust level that doesn't exceed your needs (allow things that you aren't using).  Medium trust is a good default for the things that you will generally want to be doing.  See the Security Guidelines from Microsoft's Patterns & Practices folks: http://msdn2.microsoft.com/en-us/library/ms998258.aspx#pagguidelines0001_codeaccesssecurity

Hope it helps,

however, be aware, that some modules and providers (e.g. OpenID) require high or full trust.