OG with yeast pitched

I know everyone says check your OG before you pitch your yeast. I'm sure there are really good reasons, but why? The only thing i can think of is that if your gravity ended up a lot higher then you could dilute it if you wanted to. Can it actually effect you gravity reading if you forgot to check until after you pitched? I'm curious cause this happened to me last weekend. I realized i had not taken a gravity reading yet right after i pitched. I took a reading then. It was 1.058. Any way to know if this reading is accurate?
Zac

 

The problem that you run into is that when you take a sample to get your reading you'll most likely pull out some of the yeast that you just pitched.

 

If you remember right away, you are fine. The problem comes when you forget and then don't take a reading for a few hours or the next day. You'll have no idea what your SG was if it has already started fermenting.

 

I always take a reading after pitching.

I transfer to the primary, pitch the yeast, let it sit ~ 10 mins, shake the bejeezus out of it, and take a reading. By the time it's all shaken up, I can't imagine I'm pulling a crucial amount of yeast out... and I see no reason why it would not be accurate.

 

Exactly why I used the word hours and not 10 minutes.

 

Right, as long as you get the sample within a few hours it will be correct. I pull samples after pitching because that way there is some yeast in the sample and I can watch the gravity drop.