i don't you'll find anyone who disagrees with what x-factor said:
The only reason you would ever not use filler rod for tubing is when you have overlapping seams, and its not structural. Neither apply for what you are asking
i've seen some guys on the tool forums say that they've been taught or told that there is no need for filler if you're doing a butt weld tight together or whatnot, but one
factor in getting a pipe weld certified(or in just making it as strong as possible in general) is that the welded area is the same thickness as the pieces going together(within certain parameters)
in a system that is going to see pressure, or in a structural situation, you DO NOT want the weld to be thinner that the base metal, which is what you would get with no filler metal....