Thank you for writing.
Telerik UI for WinForms supports two versions of .NET - 2.0 and 4.0. The latter includes a few more components (like RadRichTextBox, RadSpellChecker) and a few control improvements due to the improvements in the .NET framework. For example RadPdfViewer will perform a bit better performance wise with .NET 4.0. There is no benefit from using .NET 4.5, so I would not suggest wasting time to build the controls with it. In addition, there are no issues with drawing or anything from using the 4.0 components with .NET 4.5 (at least so far we have not encountered any, nor we had such reports).
I hope that you find this information useful.
Check out Telerik Analytics, the service which allows developers to discover app usage patterns, analyze user data, log exceptions, solve problems and profile application performance at run time. Watch the videos
and start improving your app based on facts, not hunches.