It's common -- I've been doing it for almost 40 years, and looks fine.

The reason folks have been doing it for "almost 40 years" is that it enhances the efficiency of capturing the effluent streams coming off the stovetop/cooktop and the cookware. The stuff from the stove-top naturally spreads conically outwards and upwards. (A hood is not a vacuum cleaner.) Making the hood wider than the stove-top or cooktop is standard practice. It will not look silly.

It's always better to have more capture area.

Thank you for the feedback and photo! This is very helpful!

Definitely better to go 6" wider than cooktop. Your builder knows his stuff. It will look the opposite of "silly" - it will have a more "pro" look, and the proportion will be very pleasing to the eye, as illustrated in the pic above.

My hood is wider too. The only problem I had was the backsplash. I wanted a stainless steel backsplash behind the rangetop with tile everywhere else. So I still don't have a backsplash! If the stainless steel is as wide as my hood, it will be larger than the width of my rangetop. If I make the stainless steel as wide as my rangetop, it will be too narrow for the hood. I know I could solve the problem by tiling everything, but I really want an easy clean backsplash behind the rangetop. Ask your builder about the backsplash.

Psyche - I am linking a thread which discusses when the hood is wider than the range, should the backsplash behind the range be the width of the hood or the width of the range. The conclusion seems to depend on whether there are upper cabinets on either side of the hood. If there are upper cabinets, then you should match the backsplash width to the hood width. Otherwise, you will have a 3" sliver on either side of the hood of empty wall between the edge of the hood and the cabinets.

Thread Called What Size for Stainless Steel Panel behind Rangetop

Yes, that is my problem, Thank you.