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If your a minnow your should definitely upvote your own post. You will get a bigger share of the payout, which depends on who votes for you.

If your a whale I think it makes more economic sense not to upvote your own posts, especially not immediately after you post it.

pls what is the difference between Minnows and Whale on steemit. i am new here.

It's just a term which people use to describe how much steem power they have. Small fish are minnows, big fish are the whales.