I've heard at least one reliable forum member over at NB claim that he got aroma contamination from one beer to another when they were hooked up with no check valves (without any beer backflow), something like a Helles and an IPA on the same gas connection.
As for the manifold, if it's made for a commercial system (sounds like it) then there will be no check valves at each connection because commercial couplers have a built in check valve.
If you're looking for a DIY, I built a 3-way splitter coming out of each regulator all with check valves for pretty cheap. The 'manifold' is a 4 port 1/4" FPT air tool splitter from harbor freight
, $2 each. Into each FPT port I screwed in a 1/4" MPT to 1/4" MFL check ball valve ($2.50 each at NB, item K125), then used the standard 1/4" FFL swivel nuts and barbs to connect to the gas lines with a nylon flare washer (item K127, $0.10 each). Pretty cheap way to go, but there are no shutoffs to each line so you have to do some thorough leak checking. You could put as many of these air tool splitters in line as you like with a 1/4" MPT to MPT nipple.