Consider the following example
\begin{equation}
\begin{split}
f = & \left( \frac{a}{b} + \right. \\
& \left. c \right) + d
\end{split}
\end{equation}
You should use \vphantom
, it makes a vertical space equal to its argument and no horizontal space:
\begin{equation}
\begin{split}
f = & \left( \frac{a}{b} + \right. \\
& \left. \vphantom{\frac{a}{b}} c \right) + d
\end{split}
\end{equation}
(I recommend \vphantom
over \phantom
in this case because \phantom
adds horizontal space that you don't need.)
For a lot of great advice on typesetting mathematics, have a look at Math mode by Herbert Voß.