I never post anything but when applying for a new job you can't really do without. A HR rep will not blink twice if you're not on Facebook. But no LinkedIn? They just won't take you seriously or think you have something to hide.
But the internal HR people often do. I've seen them checking my account after I applied somewhere.
The problem with the application process is getting past the initial HR. They don't usually give all the CVs to the hiring manager. This is the part I worry about.
It depends on the company and the HR person of course but I know one and they told me it's far better to have one.
All the recruiters are on LinkedIn and they are all too willing to accommodate whatever previously unthinkable salary you can think of. The fast feedback loop ("no, no, yes") among dozens of recruiters flooding your inbox ensures that you know within a few short weeks what the market can bear. You can use this knowledge to bootstrap yourself to the next tier of positions and salaries what might have taken you a decade of "natural" progression without LinkedIn.