I can say, Yes, watch ALL of SG1, and ALL of Atlantis. This new series does fall back on knowledge of former series to fill in gaps.I'm just not sure guys. I've never watched Stargate before. I've seen the movie, many times... but I've never watched the series - any of them. I was so stuck on BSG that I needed a Sci-Fi fix, and a co-worker suggested I check out SGU. So I did, and I REALLY liked the first episode... enjoyed the second and third... and then things kinda got... weird. They got really good again with "Time" but then this last episode... yeah, I dunno. I talked to the co-worker about the episode, and the fact I hadn't seen any other SG aside from the movies, and he suggested I go start watching SG1 and SGA and check back in with SGU after I finish SG1 and a chunk of SGA. I'm just not sure... maybe it's the fact I'm unfamiliar with the canon, but I'm just quickly getting bored with SGU... I wish they'd do more with the ship, technology, and other planets, aside from the soap opera that they seem to be focusing on!
I really don't think those previous shows are all that relevant to SGU. All you really need to know is the history of the Ancients, which you can learn easily enough without watching over 10 seasons' worth of TV!
I've seen a fair amount of SG-1 and a little bit of SGA, and other than a few character appearances and some Ancient lore, there's very little from the prior series that spill over into SGU that would make it confusing for new viewers.