I think it depends on how much cooking and what type that you do. I think you can get away with only a 30inch if you have cabinets on the side that trap the steam and gasses from escaping.

We did a remodel of a kitchen about 8 years ago. Installed a 30" gas range under a 30" Kobe hood. The folks who bought that house from us recently sold it - and the real estate agent touted the "recently remodeled", "high-end" (granite and stainless steel and natural maple) kitchen that DW and I put in. The point being - it held up just fine with a 30" hood over a 30" range. Is it "optimal" - probably not. Is it sufficient - absolutely. 100%. If you need the space - I'd take it. The hood will be fine at the same width as the range as long as you're not installing a fire-breathing commercial wanna-be. Even without a hood - I'd be shocked to see heat damage from normal cooking on a mainstream gas range.

I have a 36 inch cooktop with a 36 inch Venta-Hood. It does a great job of removing heat, steam and all cooking smells. There are many on this forum who highly recommend that the hood be larger than the range. I don't think it is really necessary for most users.

Contact the cooking appliance mfg. and ask their recommendation for the correct CFM for the venting appliance.
Generally, a venting appliance wider than the cooking appliance is recommended when high BTU commercial appliances are installed.

I went with a 36/36 combo and hope it works.
I am installing the Broan Evolution as I needed to keep the CFMs down under 400 for our building codes and this should be adequate for our induction cooktop - but not installed yet to see if I made the right decision.

Thanks everyone. I'm surprised by the responses. I expected more people to recommend a larger hood. I don't do a lot of frying. I make stir frys and blackened and seared fish about once a month, but that's the most smoke any coking I do would generate.

I don't plan to buy a high end range. I'm considering a mid-priced, duel fuel range for about $2,000. There's a Bosch I like and the strongest burner is 16,000 BTU.

Snoonyb, checking with the manufacturer is a great idea.

If you do any high heat cooking, you do absolutely need the hood wider than the cooktop below it. Otherwise, the sides of the upper cabinets next to the cooktop can become damaged over time. There's a pic on here of a thermofoil cabinet next to cooktop peeling, and while a painted or stained cabinet won't do that, repeated exposure to high heat can scorch it. Wokking counts as high heat cooking, even if done infrequently.

GreenDesigns, do you know how I can find a pic of that cabinet?