No you don't need to run the fan on high most of the time. I have a 5 burner DCS and a 600 CFM hood and rarely run it on high. Most of the time I run it on low. The specs are telling you the CFM recommendation for the hood in general. It's also not exactly a question of how many burners you're using at once but what you're cooking and how much smoke, steam, etc. you're generating. You should be able to see whether the hood is capturing most of the stuff or whether a lot of it is escaping. Just use good judgement.

The selection of the hood on high, medium or low has little to do with how many burners you are using. What is important is what you are cooking. If you are searing a steak with a cast iron pan on one burner, and not using any of the other burners, you will likely need to have the hood on high. Conversely, if you are using all the burners, but they are being used to boil water, steam vegetables, and scramble eggs, you can have the hood on low. Your nose and your eyes will tell you which setting works best for what you are cooking at that moment.

The specs for the range instructing a 600 ccm hood is to prevent people from purchasing underpowered hoods (people like to spend $$$ on a powerful range, and cheap out on the hood - we see it on the GW all the time). Or worse, an over-the-range microwave over the hood. It is always better to have an exhaust that you can run most of the time on low or medium, rather than, say, a 400 cfm hood that you'd have to keep on the high setting all the time. It will be much quieter too.

What everybody else said.

You are over-thinking this. Our college educations do this to us.

The DCS "specs" are for the least powerful rangehood that DCS recommend for use with that stove, as akchicago says. As Weissman and akchicago (and now me, too) say, it is not the number of burners you have going but what you are cooking that determines the rangehood setting. You run the hood at the speed that is needed for what you are cooking regardless of how many burners you are using at the time.

If you are holding various kinds of melted chocolate in six 5" sauce pans with all six burners going at once --- and I've seen my neighbor do this with four burners when making the most outrageous chocolate truffles you can imagine --- there will be no need to even turn on the hood. Yeah, you would have all six burners going, but there will be neither much heat nor any vapors to dissipate. OTOH, if you are into gonzo searing where you have one single pan heated hot enough to melt lead . . . well, you will probably need to open windows, use a floor fan and run your rangehood on high even though there is only one burner going.

So, we have a unanimous vote here that you are okay running the vent at whatever speed works best for what you are cooking.

Feeling reassured?

Thanks everyone for your replies. I was under the impression that the venting requirement was to prevent carbon monoxide. If it's just for smoke and steam then I can easily make the judgement call.

Gas ranges do not produce carbon monoxide, unless they are very very poorly adjusted. The flame colour tells you whether that is the case or not. Blue flames indicate complete combustion, whose only products are carbon dioxide and water. If you have a lot of yellow and orange flames, maybe you should clean your burner heads or arrange a service call.

Venting is to remove nasty particulate matter like smoke and grease. The more powerful the cooking appliance, the more nasty particulate matter is produced, regardless of the fuel used. Gas ranges also heat the air, which folks in warm climates like to vent, which is the only reason why gas ranges may require more venting capability than electric ones.

Make-up air may be required if your vent removes more air from your house than is being replaced by other means. If MUA is not being supplied, then your kitchen vent could pull air into the house down the chimneys being used by your furnace and/or water heater to vent their combustion products. This is called back-drafting, and THIS is how and where dangerous carbon monoxide could be introduced into your home.