Tipping has not noticeably affected my shiny encounters.
The last few shiny pokemon I have found, I don't recall being in a position to dole out tips during those sessions.
Generally I do tip when given the opportunity, but that isn't very often lately since I mostly start up, hatch/collect a few eggs and check on my berries and occasionally safari hunt.
Tipping most likely doesn't hurt the odds, but don't be surprised if it doesn't help either.
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