I've not seen any detail on this, but my guess would be that a few mutual likes would be fine, but if all your likes were mutual then Google might start to question how genuine they were. But for odd ones here and there, each like is so insignificant in the final count that I don't see it is worth worrying about. But it's a good question to ask!
Any posts I write in Arial are on my mobile phone, so please excuse typos etc. Any posts I write in Verdana are on a PC, so feel free to berate me mercilessly for any mistakes