I work on a lot of different brands, models, and different equipment. Yes if I can find a service manual I go by the torque specs in it. I also find I don't have that luxury on everything I work on. What I have done is made up a written torque chart with all different sizes and grade hardware. I have used this chart a lot when working with aluminum housings and head gaskets. Also a lot of the common sizes I just remember. By doing it this way I have never had any problems like striped outs unless the threads are already bad. I have found on hardware that a nut is on the other end or spark plugs I just use common sense and can tighten by feel. Just out of habit for some reason I alway torque wheel nuts, studs or bolts?