I've always imagined it was less the Emperor himself keeping humanity safe from corruption, and more the tenants espoused by the Imperial Cult. The Chaos gods draw their power from the base impulses of sentient beings. First you give yourself over to the brutal Ecstasy of violence, THEN you start worshiping Khorne.
The Imperial Cult forbids the sort of hedonism that Slanessh promotes, and while it glorifies violence, it only does so to the degree that violence serves a higher cause (A true Khornite dosn't care whose head is getting bashed in, so long as heads are bashed). An Imperial will look at the aftermath of an artillery strike and say "Look at all those heretics that just died. Today was a good day". A Chaos Worshipper will look at the aftermath of an artillery strike and say "Look at all those people who were blown to bits, today was a good day". An Ork will look at the aftermath of an artillery strike and complain that he missed the show, followed by a demand that they do it again.
Really, any sort of system that discourages the whims of chaos will do. The Gue'vesa are just as devoted to the Tau and the philosophy of the Greater Good as the Imperials are to the Emperor.
Also, Corruption =! Falling to Chaos. A guy who enjoys getting drunk and beating people up may be corrupted, but he's a long way from shouting "Blood For The Blood God". In fact, if he gets himself a uniform and limits who he beats up, he could be considered a model citizen. So just because a planet dosn't have the Imperial Cult keeping everybody in line, dosn't necessarily mean it will fill up with demon worshipers. It takes something very extreme to make somebody a chaos worshiper, that's why Cults of Slanessh throw drug-orgies, rather than just sitting around eating chocolate.