Is it worth to premine all new cryptocoins?
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Not sure if it's going to be a waste of time or not
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You mean "mine", not "premine". It's called premine only when it's mined before it's announced to the public.
Nobody knows what could become worthy. I mined a couple of Dogecoin blocks when it was released just for the lulz and then stopped. Turns out I had mined what now is valued at 0,5 Bitcoins in 30 minutes. I could have made much more had I continued mining DOGE.
On the other hand, I invested significant mining resources on the coin known as Offerings to Cthulhu and there's not even an exchange for that yet.