Although it is possible that no version of electrum will work on the "accepted" fork in the event of a fork, the private keys that will get exported will not change depending on the time that  you export them. In other words, you will incur the same output if you export the private keys today, tomorrow or 10 years from today (you will likely receive additional keys the later you export them because the chances are that you will use more addresses as time progresses, however this fact should be ignored for this argument). As a result it does not matter when you export the private keys, as if the keys are valid for the fork you wish to use, then they will always be valid regardless of when they are exported.

As you somewhat pointed out, holding an unencrypted version of your private keys is very dangerous, and many bad things can happen to them, and the risk of loss is significantly greater if you are storing an unencrypted version of your private keys somewhere (especially if you are storing them in some cloud storage service).

I would say the best practice would be to store you seed and/or backup of your wallet in a secure manner and then only export your private keys in the event that you absolutely need to.