If you use a locked-out derailleur as a chain tensioner, chainline does not have to be perfect, but derailleur alignment does. Derailleurs are designed to take a multi-speed chain in at an angle and place it properly aligned on the cogs. There is probably a slight loss of efficiency in the chain if it is being pulled toward the chainring at an angle, but if you have a 'close-enough' chainline (within 5mm or so), this effect will be minimal. If you try to fudge chainline with a single speed chain or while using some chain tensioning method other than a derailleur or derailleur style tensioner, you will not like the results.