It is fine to use Lisp. It is generally not
advisable to use it without knowing what the
functions you use do (or don't do).
That's one way to learn, of course, - by
experimenting, but it can also help to read
the fine manual.
For setting single variables, one would think
using common sense, and the kind of common
sense that comes with using computers every day
and evermore pushing the envelope to increase
one's understanding, it would seem this would
be enough, at the very least if the docstring
(help system) also is consulted for the
specific setting. But reading the manual is
always encouraged, of course.
If the docstring isn't enough, but the manual
is, one can report it as a bug and suggest how
the docstring can be improved.
Yes, the doc string for such an option should warn you
not to just use `setq'. But not every doc string is
perfect. ;-)
The Elisp manual, node `Variable Definitions' says:
‘:set SETFUNCTION’
Specify SETFUNCTION as the way to change the value of this option
when using the Customize interface. The function SETFUNCTION
should take two arguments, a symbol (the option name) and the new
value, and should do whatever is necessary to update the value
properly for this option (which may not mean simply setting the
option as a Lisp variable); preferably, though, it should not
modify its value argument destructively. The default for
SETFUNCTION is ‘set-default’.
If you specify this keyword, the variable’s documentation string
should describe how to do the same job in hand-written Lisp code.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
http://www.gnu.org/software/emacs/manual/html_node/elisp/Variable-Definitions.html
Note the second paragraph. If you define an option with a
`defcustom' that uses `:set', help users of your option by
letting them know how to change the value using Lisp. How
to change the value typically does not mean using only `setq'
in such a case.