On the MX side I always understood it as it would re-mark anything that doesn't have the correct marking already, outbound. However QoS on the Internet doesn't exist, so the only real benefit to doing this would be for when the traffic goes over the VPN tunnel (if you have AutoVPN running).
The only thing that I know that does (or can?) affect the Internet destined traffic would be the specifically the 'priority' option as it would drop into a different 'bucket' for the MX to process in priority of high to low kind of thing.
To be fair though I've never really messed with the priority setting stuff, but I read up on it and if memory serves just concluded it wasn't worth the hassle, I think mostly because it also requires that you have the bandwidth configured for upload/download accurately (which as we all know DIA circuits are never 'exact' anyhow) on the traffic shaping settings for the MX, so that it can correctly allocate the right percent of 'known' bandwidth and adjust the buckets accordingly.
Even then I'm still sure I got it wrong, therefore the 'not worth the hassle' part for me just cropped back up again lol