Do a little research. This comes up over and over again here.

Here's one place to start:

Help with ventilation

6" extra is recommended but not mandated. If you have a wall hood between cabinets it's less necessary. Many of us have hoods that are the same size as the range. IMHO, the depth of the hood is more important than the width.

Capture size is everything.
The bigger the capture size the less steam or smoke, depending on your cooking ablilities, will escape into the kitchen. This is why most range manufacturers suggest going to a little larger hood than the size of the range.

As smoke & steam rise from the pans they expand and get more volumous. The higher the hood sits off the range the larger the capture size should be. Sometimes capture size can be artificially influenced by using a higher volume blower but this can also mean louder noise levels.

As stated by others the ideal is 3" wider either side - most installs are the same size in my experience otherwise the wall cabit design doesnt sink up with the base cabinets. In addition to height above range and placement of hood, cross drafts from other air flows like heating or air conditioning) island or on the wall placement between cabinets affect the efficient operation of the hood. There are other factors to take into account as well - cfm required (about 1cfm for every 100btu of max cook top output), lenght of duct run, baffle filters vs mesh. If all factors are perfect you can get away without that extra width.

This question has been answered before but here's the quick answer. If it's a standard wall install (not island or peninsula) and you have cabinets on either side, then 36" for 36" range should be fine. Just be prepared for a bit of cleaning on the sides and underneath the cabinets if you make something with lots of grease smoke. The back wall and cabinets provide a trap for the smoke that gives your hood the ability to capture and pull it out, hopefully, before the grease condenses on surfaces. You might want to consider at least a 900cfm unit rather than the more standard 600cfm for that size since it may need to work harder to pull smoke out faster. This is especially the case if you have a grill on your range. Other thing to look at is overhang. You probably do want to make sure the front edge of the hood comes out 6" beyond the front vertical plane of the range.

You mentioned cabinets so it doesn't sound like you have an island or peninsula but in those situations, you'll want at least 3" on each side since there's nothing to guide the smoke.