Do you remember when you were a child, and the teacher used to scold you for not reading the instructions?
Well, read the instructions...
Anyways, I've extracted this bit from Contributed to EOS with MyEtherWallet - The Complete Guide last week to make sharing this information on the telegram channel more accessible. I will be maintaining this and eventually it may diverge from the version in the original article. I have declined payout since this is mostly a copy/paste deal
About Registering a.k.a Mapping your EOS Public Key
What is it?
Registering your EOS key will map a public key to the Ethereum Address this function transaction is broadcast from. It is very important to understand this, because 48 hours after the token sale ends, ERC20 tokens can no longer be transferred, and where they live at that moment in time will be where they die, only to be reborn on the EOS blockchain in the genesis block according to the mapping defined by this function.
(takes deep breath)
For example, if you move the EOS ERC20 tokens to an exchange, and fail to move your tokens back to an Ethereum address you posses the private keys for that is mapped/registered to an EOS address, you will not have access to those EOS tokens on the native chain after June 3rd 2018.
...But relax, you have time, there is no rush or need for anxiety yet, June 3rd is still a ways away
Does it matter when the EOS keys are mapped to an Ethereum Wallet?
What's most important about all this is that you have a valid EOS key-pair whose public key is mapped to an Ethereum wallet containing EOS ERC20 tokens BEFORE the end of the 48 hour grace period following the end of the EOS token sale on June 1, 2018 at 22:59:59 UTC
This means that registering a.k.a. "Mapping" your EOS key can be completed at your discretion, before or after funding the EOSCrowdsale Contract, at any time between now and the end of the ICO.