Dougs headers are among the good ones and if dougs says you should use some RTV then sure follow it they know there stuff,the only bad thing i ever experienced with the UltraSeal gaskets are that they sometimes becomes very attached to the parts they are supposed to just seal and maybe that will be less of a problem following dougs instructions.
My coment about not using anything but the gasket is based on seeing to many people bandaiding warped flanges by using a stack of gaskets and all sorts of weird sealers.
Another possbility is that headergaskets can develop a leak if you have plenty of power and a very restrictive exhaust but thats a rare problem.
sometimes you just need to retourque the headerbolts after a while and sometimes they seem to stay put with no isues almost forewer.