In my humble opinion, chain line should be within ~2mm for single-speed, and within ~1mm for fixed.
You only have one gear, so the chain takes a beating every time the road goes upwards. To withstand that level of use, the chainline should be perfectly straight, as that's when the chain is strongest. If it fits, I'd run a proper single-speed/fixed chain, with a three piece link, rather than a regular derailleur chain.
Please explain to this thicky how chain tension varies as you go up hill, when compared with multi geared machine. I would see it as the chain taking the same load whatever gear you are in.