Setting default overlay specification for itemize environment

You can \renewcommand{\itemize} and set the default overlay specification (after \begin{document}):

\documentclass{beamer}
\usepackage{letltxmacro}
\LetLtxMacro\olditemize\itemize
\begin{document}
% Update itemize to have a default overlay
\renewcommand{\itemize}[1][<+(1)->]{\olditemize[#1]}

\begin{frame}
Some question ...

(a) Part a of question 
\begin{itemize}
  \item Step 1 of solution
  \item Step 2 of solution
\end{itemize}

(b) Part b of question
\begin{itemize}
  \item Step 1 of solution
  \item Step 2 of solution
\end{itemize}
\end{frame}

\end{document}