Informal poll time: anyone on this list who has participated for 10 or more years in a 401(k) who isn't up wildly despite this past year? Let's call "wildly" something vague like total assets more than double total contributions and forget about the tax issue.
I mean: 10 years ago, the DJIA was around 3000.
But sure: some years there would be a "negative rate" despite positive contributions. For sure if you _started_ this year, you're likely to have seen an immediate drop (though perhaps not even enough to have you worry about it post-tax ...).
/jordan