The answer, as with all things in starting a business, is that you have to make wise decisions that yield good results. Seriously.
If you choose a great co-founder with complementary skills and personality, it will drive you forward. If you choose poorly, you would have been better off staying single. The only thing that is really going to matter is your specific results. There is no reason for there to be a general rule that applies to everyone.
Single founder success stories are the minority. Despite the leading cause of startups failing being founder breakup, the greater chance of winning still lies with having one (or two) cofounders.
So what's the "right" thing to do?
Have a cofounder to greatly increase your chances or success, or endanger your company by getting a cofounder?