I disagree that .form-group should be within .col-*-n elements. In my experience, all the appropriate padding happens automatically when you use .form-group like .row within a form.
<div class="form-group">
<div class="col-sm-12">
<label for="user_login">Username</label>
<input class="form-control" id="user_login" name="user[login]" required="true" size="30" type="text" />
</div>
</div>
Check out this demo.
Altering the demo slightly by adding .form-horizontal to the form tag changes some of that padding.
<form action="#" method="post" class="form-horizontal">
Check out this demo.
When in doubt, inspect in Chrome or use Firebug in Firefox to figure out things like padding and margins. Using .row within the form fails in edsioufi's fiddle because .row uses negative left and right margins thereby drawing the horizontal bounds of the divs classed .row beyond the bounds of the containing fieldsets.