I answered the first time more from the perspective of how far I am willing to drive. I see others posting more from the perspective of overall territory / preferred geographic areas, so I figured I would weigh in on that as well.
When I started chasing in 1996, it was with Marty Feely's Whirlwind Tours, one of the first tour groups. Marty had firm rules, similar to what
@Warren Faidley articulated. That's the way I first learned, so I have stuck with that, for the most part. I feel like that territory, mapped out by the good visual that
@Jason Boggs posted, is "old school"; back then, people just did not seem inclined to chase east of I-35, but that could just be my perception. In chasing today, it seems that no place is off limits, and this is not a good trend as far as safety is concerned.
I stretch the boundaries of Jason's map somewhat, and I'm sure he and Warren do as well. It's hard to forego a good setup just because storms are going to initiate or move a little east of I-35, especially if it's the only option. If there are options in better terrain, then it's an easier decision to turn away from the east-of-I-35 target. I missed Joplin in 2011 because of my general rule to not go east of I-35. I also avoided the Lawrence KS area on May 28, 2019. Neither day turned out well for me, although on May 28 2019 I had only myself to blame for missing the even better tornadoes in Waldo and Tipton.
Also relative to Jason's map, in 20+ years of chase vacations I have never been as far north as North Dakota (probably not much if any further north than I-90 in South Dakota), have never been in Montana, and have only gone as far north in Wyoming as Casper. I've only chased in Iowa, Illinois, and Missouri once each. This gets back to my post above, of setting limits on how much I am willing to drive, even though I'm on a chase vacation.
I would expand Jason's map slightly in one place though, southeastern Nebraska is pretty flat and has a pretty decent road network.