If you use regular LaTeX with the OFSS, then you will probably get
incorrectly sized letters if you use \bf in a superscript. For
example, both letters in the output from ${\bf x}^{\bf x}$ are the
same size. This problem has been corrected if you run REVTeX on the
OFSS.
There are also problems if you try to get bold math symbols in LaTeX.
The solution given by the LaTeX User's Guide & Reference Manual is to use \mbox{\boldmath$#1$}
where #1 is the symbol to be set bold. There are problems with this
approach. On the average LaTeX setup with the OFSS you only use this for
\cal, lowercase Greek letters, curly brackets, and other
miscellaneous symbols. You will not be able to get these characters in the
correct size in a superscript, either. If you use the NFSS you will also be
able to set digits, uppercase Greek letters, parentheses, and square
brackets in boldface using \boldmath, but you will still not get
them in the correct size for superscripts.
The REVTeX command \bbox{#1} will make #1 bold in math
mode, but it will first make sure that it is the correct size, even in
superscripts. If the correct font in the correct size is not available then
you get #1 at the correct size in lightface and LaTeX will issue
a warning that says ``No \boldmath typeface in
this size . . .''.
So the \bbox{#1} command will give bold output of #1 in math
mode. You can use it to get bold greek characters---upper- and
lowercase---and other symbols. It is still easier to use \bf to get
upright Latin letters in boldface. How much comes out bold and in how many
places you get bold output depends on how many fonts you have installed.
Using the amsfonts option will automatically use the extra AMS
Computer Modern math and symbol fonts for bold in superscripts and smaller
sizes, if you have installed the AMSFonts. The following will come out bold
in bboxes: normal math italic letters, numbers, Greek letters (uppercase
and lowercase), small bracketing and operators, and \cal.
If you use only on-line bold math symbols there is no advantage to using
\mbox{\boldmath$#1$}. If you use superscript bold math symbols then
you need the AMSFonts installed and the amsfonts style option to see
bold, but the symbols will be in the correct size and will come out bold at
the APS. Overall, it seems better to use \bbox{#1} everywhere.
Note that \bbox{#1} is a fragile command.