# \centering vs. centering environment

It recommends: Note that (1) figures should be centered via the LATEX \centering command—this is a better approach than using the center environment which adds unwanted vertical spacing;

But LyX will use center environment if you have a graphic in a figure and choose center paragraph alignment.

You can, of course, just insert \centering with ERT.

Shouldn't LyX automatically use \centering as recommended when using IEEEtran?

The centering of content is generally considered an element that is independent of the document class in use, since you can still use the center environment if you want. Also, it is not specific to ieeetran.
When I issue Edit > Paragraph Settings... > Alignment > Center for an image inside a figure, LyX places that image inside a centering environment, not the center environment. Yes, there's a difference.
Using the environment form of a macro (or switch, like \centering) is not problematic per se, since \end{centering} results in \csname endcentering\endcsname and even though it - \endcentering - doesn't exist, it doesn't generate an error (see section 11.6 Construction of control sequences, p 116 of TeX by Topic). Moreover, LyX ensures that \centering (albeit in its environment form) has an ending \par; necessary for setting the paragraph formatting.
The advantage of the environment form here is that the scope is limited to within the \begin ... \end clause.