You can wait to pitch your yeast but you must keep in mind that wort is a prime environment for microorganisms to flourish. In brewing beer, the only microorganism we want in our wort is yeast. If your sanatation methods are not up to par, you will invite some nasties to make a home in the wort and cause undesirable results ranging from off flavor to infections.
Some brewers will make a real wort starter if they are not chilling after the boil. This will require the wort to wait 24-48 hours after the boil until yeast is pitched. I've tried it once when I found myself with an unexpected brew day and no time to make a starter. Everything worked out fine.