So to my understanding the reason that you want to turn NoScript on (or leave it on in TAILS) is because a malicious script could reveal your true IP. If you only ever log onto a certain Reddit account through TAILS, is it possible that whitelisting Reddit will reveal your identity? I'm curious about this in light of the canary clause being taken out of Reddit's transparency report, and the very real possibility that LE is requesting user info.
If you whitelist any site to be allowed javascript and or flash then yes that site can deliver you a exploit that can reveal your true IP. Without the site serving you malicious content the answer is no. The deal with reddit and the NSL as far as we, anyone who's been digging to get to the bottom of it, can tell was just for user information.