it's hard to say whether to withhold more - that is a personal decision . if you have owed in the past, then it is likely you will owe more next year, assuming the earnings are the same, and more of the income is tilted towards salaries and not towards bonuses.
taxwise, it really doesn't make a difference which paychecks the 401k money is pulled from.... what is probably more importnat from a financial planning aspect is to maximize the contributions if the company is matching,
