If you measurement is very good, you might have a lot of decimals before
the '' finally appears.
Compare
8°34'12''.12343
to
8°34'12.12343''
i think the former looks a bit better, esp. if the decimal point is
moved _under_ the arcsec symbol. But i guess it's a matter of taste. ;-)
It'd be nice if siunitx had a switch for that ... and of course it has:
\ang[angle-symbol-over-decimal]{45;12;23.22}
Used as a package option, it doesn't change the output of
\SI{12.3}{\arcsecond}, however.
Karl