Steem is decentralized and there are no specific rules that say you should not upvote yourself or that you should.

Exactly. The nature of the blockchain means that issues (at least most of them) shouldn't be a question of whether we should or should not do something, but that of preference. And when it comes to preference, there is no right or wrong. Of course, once the community weighs in, and a consensus is reached, things take a different turn.

So, until that consensus is reached, whether by the witnesses making a categorical decision on things (I assume it is one of their jobs), a blockchain update penalizing or banning certain actions, or some other consensus-building initiative, I say self-upvote all you like. Or perhaps, self-upvote with caution? After all, as you pointed out, no one wants to see poorly written, spammy posts self-upvoted with abandon.