One of the most precious commodities that you have while dealing with IRS is time. You might owe them a lot of money - but you can accidentally accelerate your case and put yourself in a deadline that you regret by calling the IRS too soon. Knowing when to call the IRS - is another reason why investing in Representation can be key.
For example - the CP504 - Urgent - We Intend to Levy letter causes a lot of freak-outs to the average taxpayer. But in actuality - this letter is one of the earliest in the collection's process. If you have an emotional reaction and feel the need to call - you might place yourself in a deadline the IRS - about 3 or 4 months earlier(!) - than you needed to.
Now - if your outcome with the IRS - is an Offer in Compromise or some other hardship procedure - there is no reason to wait per se. But if your outcome is going to be a payment plan of some kind - well - you might as well take advantage of every month you can - before paying them.