jQuery toggle CSS?

I would use the toggleClass function in jQuery and define the CSS to the class e.g.

/* start of css */
#user_button.active {
    border-bottom-right-radius: 5px;
    border-bottom-left-radius: 5px;
    -webkit-border-bottom-right-radius: 5px; /* user-agent specific */
    -webkit-border-bottom-left-radius: 5px;
    -moz-border-radius-bottomright: 5px;
    -moz-border-radius-bottomleft: 5px; /* etc... */
}
/* start of js */
$('#user_button').click(function() {
    $('#user_options').toggle();
    $(this).toggleClass('active');
    return false;
})

For jQuery versions lower than 1.9 (see https://api.jquery.com/toggle-event):

$('#user_button').toggle(function () {
    $("#user_button").css({borderBottomLeftRadius: "0px"});
}, function () {
    $("#user_button").css({borderBottomLeftRadius: "5px"});
});

Using classes in this case would be better than setting the css directly though, look at the addClass and removeClass methods alecwh mentioned.

$('#user_button').toggle(function () {
    $("#user_button").addClass("active");
}, function () {
    $("#user_button").removeClass("active");
});

You might want to use jQuery's .addClass and .removeClass commands, and create two different classes for the states. This, to me, would be the best practice way of doing it.