it is better to renew it earlier if your GC is not coming soon

I think that you'd better renew that earlier if you know that your GC is not coming the time your EAD will expire. Although EAD is fast, you never know what is really going to happen to you. So it is better to renew that 3 months prior to its expiration date.
And your expiration date of your renewed EAD is not going to be changed by the date that you renew that. It will be effective starting from the date that your old EAD expire. So you are not losing anything by renewing that earlier.
Also, INS is increasing the fee for everything. It is better to save some money.