Like other's have said the best thing is to just experiment and see what you like. The nice thing about the HomePod is you don't have to wire it to a receiver so you have a lot of leeway for placement. I think it needs room to breathe and would sound better with walls to reflect off of. Someone had this in the middle of the room and noted it didn't sound as good and I'd agree giving it some wall close by to reflect off of would be best.
If you put it in a corner the bass will get stronger. Some people complaining about boomy bass may be doing this (they may not but this is the biggest cause of boomy bass). Anywhere you put a speaker is going to have peaks and nulls, the HomePod gets away from this in the upper range through reflections and the sheer amount of drivers it has but there's still one woofer so where you place it will affect the bass response. Small movements can have a big impact on this so it's good to just try out a few places and see what sounds best. This is also something where everything in your room impacts this, do you have carpet, how much do you have on the walls, size and shape of the room and how much furniture you have in the room so there are general guidelines but every room is different so there is really no shortcut to trying out different locations.
If you have multiples in one room spreading them out would be best so they can even out the sound in the room. For like a TV you'd want one under or one on each side. Someone had a problem because they had to put one in their cabinet which would really kill the sound since I think the main benefit is it reflecting large sound so they may be better off putting a riser under the TV so it sits higher and then put the HomePod in front of it the TV on the stand but below the screen so it can reflect sound.