The other thing you need to look at is how and if your company matches any funds. My company matches 50 cents on the dollar up to 6%. In other words they put in up to 3% for every 6% PER PAYCHECK! So if I reach my max contribution in a July and then don't contribute in Aug-Dec they don't contribute anything in Aug-Dec.
I put about 8% per check to my 401K each pay period and reach the max contribution threshold (hence "max out") in December hence maximizing my firm's contribution.
The only way you are truly "maxing out" your 401K at a 20% contribution rate is if you make 82,500 per year. (82,500 * 20% = 16,500)
If you make more than 33,000 and your company matches PER PAYCHECK, then it is ill advised to contribute 50% since you would max out your personal contributions before the end of the year. (33,000 * 50% = 16,500)