yes you do. the stock cogs on bikes direct bikes are unfortunately cheap. (and the threads on the hubs are also of questionable quality, despite that theyre supposedly rebranded (or de-branded/blank) formulas.

when riding fixed, drivetrain is something you definitely dont want to skimp on. getting top of the line cog/lockring (dura ace, EAI, etc) is only ~$30 total and a relatively minor expense. you'll be glad you did.

edit: ahem, i did say "etc," but yeah surly cogs and lockrings are good, too. you get the idea; dont cheap out.