Checking the box is always optional. However, I recommend doing it for clerical purposes. You will have to explain your reasoning of why you consider yourself disadvantaged. Nevertheless, it will be a very good opportunity to restate what you just wrote.
Sometimes, schools don't ask for any details. Sometimes they do--in which case they will ask things like if you have ever been the recipient of funds from any federal means-tested programs, such as Free and Reduced School Lunch, or state Children's Health Insurance Programs (CHIP). I would say that if you are a Pell Grant recipient, this would probably qualify you, as the rules for Pell Grant allocation are pretty stringent.
I would say that you definitely qualify, but what it comes down to is how confident you feel about being able to justify your answer if you do check the box.
There are absolutely people who are poorer than you and didn't check the box because they grew up happy and never knew any different, and there are people quite a bit richer than you who did check it because their parents couldn't buy them brand name clothes every year for back-to-school and they *felt* very disadvantaged. (no, I'm not saying this is all people by any means)
I kinda wish they could make it more objective - like that it applied to everyone who grew up within x% of the poverty level in the county where you lived or something, and that's it. They could still ask questions about how disadvantaged you felt in interviews, but as it is it seems like it has at least as much to do with the personality of the person and how entitled they feel as with their actual economic situation. The happiest person I know grew up very poor to a single mother, but she would never consider herself disadvantaged in any way. But I was a brat when I was a teenager and except for the fact that my family was above the income cutoff, I'd probably easily be able to justify being economically disadvantaged, because my parents didn't buy me tutors or fancy SAT classes or anything like that. And I had to get a job when I was 15 for extra spending money.
Also, they probably will never bring it up or ask you about it unless you write about it in your personal statement / secondaries. Maybe some adcoms can weigh in, but I haven't gotten the impression that they put economically disadvantaged people in a special pile for consideration.