Of course you should opt into EL sales for all your images. You have nothing to lose, only the chance of gaining an extra commission for a sale.
I've had two EL sales at FT so far this month giving me an extra $74 making it a BME. I recently turned silver and upping my EL price to the maximum 100 credits has been worth it. I don't think keeping your EL credit price lower than the maximum allowed impacts EL sales.
I recommend opting into ELs on all your images on all your RF sites.