I was talking about adages and was laughing at that one, but then I realized that I don’t know a quaint or antiquated version of it…what’s the “a stitch in time saves nine”-style adage that means “don’t shit where you eat?”
The version I seem to remember, though poorly perhaps, is “Don’t get your meat where you get your potatoes.” or something like that. Maybe somebody has a better version.
Don’t dip your pen in company ink
Don’t soil your own nest?
Don’t pooh where you stew?
Don’t defecate where you consumate?
Don’t stool where you drool?
Ok, I got nothing.
But I too look forward to more answers on this one.
Don’t get your sugar and your bread at the same store.
Don’t shit on your own doorstep.
Or Don’t poop on your own doorstep if you want it polite. 
Don’t defecate where you masticate.
Don’t get your meat where you get your bread.
Don’t get your honey where you get your money.
Don’t grunt where you hunt?
And here I always thought it was “. . . in your mess kit.”
Don’t lather where you gather.
Actually, the pen in ink is probably where you’re going.
My old boss at the golf course had a charming saying for this:
“Don’t fuck the help.”
People shouldn’t throw stones if they live in a glass house? That’s a similar sentiment, but not quite right I think. There is an older version of the sentiment, but I’m not bringing it up yet.
I think the Southern version is Don’t mess in your nest.
Don’t fish off the company pier.
I’m having trouble picking up the metaphor in that one. Could you explain it? 
Control the Hobbit
Or meet Ms. Bobbitt
“It’s a dirty bird [indeed] that fouls its own nest.” There are a lot of versions of this one in a number of different languages.
Though I wouldn’t be surprised if “don’t shit where you eat” is quite old.
I think that’s the one I was trying to bring up, or close to it.
“Don’t bring up what you’re close to.”