It depends. On most farm maps I place the mushroom log grove on the farm, wherever it seems to fit. Sandy part on the Beach farm. West of the big pond on the Forest farm. Same landmass as farmhouse on Riverland farm. And so on.
On the Hilltop farm map, there's not enough space for a decent grove, so I put it up at the railroad, marking the path Sebastian likes to walk with different paving so I knew to not put any logs there... again.
You could also put it in the quarry, split it up around the mountain lake, or even put one on Ginger Island once you've got access to it. I would recommend placing an indicator log on your farm if you have the grove off the farm. Indicator tapper trees if you're tapping the trees in the grove as well and want to make sure you don't miss that harvest too often.