Given the scale of previous eruptions, you'd need to cross off the entire western half of the U.S. In the unlikely event that Yellowstone explodes, it wouldn't matter much whether you were in the park or 350 miles away.
Anywhere on earth will get you killed if it blows.
Late Pleostocene Human Population Bottlenecks. . . (Toba)
"The six year long volcanic winter and 1000-year-long instant Ice Age that followed Mount Toba's eruption may have decimated Modern Man's entire population. Genetic evidence suggests that Human population size fell to about 10,000 adults between 50 and 100 thousand years ago. "
I think that most scientists believe that the eruption, when it occurs, will have world-wide implications. I don't think you'll be able to hide much of anyplace.
It's a good thing I live 10,000 miles away in the South Pacific... Hmm, the resulting tsunami might be a problem though.
The extreme western portion of the U.S. near the coast would fair much better then Idaho, eastern Washington, Nevada, Montana, Utah, Wyoming and just about everything else downwind, east and southeast of Yellowstone. Good luck.
IIRC the las one was what made our plains states so fertile. The heavy ash fallout went nearly to the Mississippi and that is farther away than 350 miles.
It would be spectacular and would make half the US super fertile.. after it was rained on for a half a century or so. Yeah it would probably kill all the fish in the rivers to the east of it, but eventually it would be better.
The earth and nature are fantastic when viewed on time scales we can only imagine.