Just how much does the parallax setting matter on a squirrel rifle's scope?

For those not as familiar with "parallax" (regarding a rifle scope), it is how much your crosshairs move OFF target when you fail to position your eye perfectly straight thru the scope tube.

Most rifle scopes (with non-adjustable parallax) are factory preset to be "parallax-free" at 100 yards. This means at 100 yards, even peering into the scope tube at an angle, your crosshairs will not move off target at 100 yards. But they sure will at 25 yards! Enough to miss a squirrel's head! Try it for yourself by moving your head slightly left and right as you peer thru a 100-yd-parallax-free scope at a target about 25 yards away.

Because of this "parallax" issue, most "target" scopes come with an adjustable parallax (often referred to as "adjustable objective"). And while most fixed-parallax scopes are factory preset at 100 yards, most "specialty" marketed scopes are not. Most "shotgun" scopes are set to be free of parallax at either 50 or 75 yards. Leupold's "turkey" shotgun scope is parallax-free at 40 yards. Nikon's Bolt "crossbow" scope is parallax free at 20 yards.

Another simple everyday example of parallax can be seen in the dashboard of motor vehicles that use a needle-style speedometer gauge. When viewed from directly in front, the speed may show exactly 60; but when viewed from the passenger seat the needle may appear to show a slightly different speed, due to the angle of viewing.

One reason I'm asking "how much" does it matter, practically speaking, is because I'm thinking of mounting a 2-7 x 35mm Burris Fullfield II on a new dedicated "squirrel" rifle. The usual targets will be squirrel heads at an average range of probably 30 yards, and seldom would a shot over 50 yards be an option. A Burris scope can be sent back to the factory to have the parallax reset to a different yardage (without cost other than shipping). This particular scope came preset at 100 yards, while I'm thinking about having it changed to 50 yards, or maybe even 40.