Are you talking about Easyjet flights? When first released, they are very cheap, but not necessarily at their cheapest.
If you book when they are released, there is a small chance that you may see the same flights cheaper on a certain day, but this is not at all certain. It is much, much more likely that the flights you see at a certain price when released will only increase in price.
If you want to go anywhere and get a very cheap deal and don't mind when you travel, it could be worth waiting.
If you know where you want to go and when, my advice is to book immediately. That way, you know what you have paid and your flight is sure. The price could go down a little; probably it will go up a lot.