There's a reason countries don't have "more modern" people on their money. (With the exception of current heads of state.) Whether or not they are currently believed to be a great person, it's usually advisable to wait more than a couple of days after they're in the ground to see how history judges them. It's the same reason they don't normally name schools or public institutions after living people. If opinion is still in favour of the person ten, twenty or fifty years later, that's a pretty good sign they're a safe bet.

Of course I might not be the right person to say this, because New Zealand took the extremely risky step of putting Sir Edmund Hillary (probably one of the two or three most universally respected men in the country) on the $5 note while he was still alive. In general I don't think it's a good idea though.