Yes, you do need to ride on I-80. See http://www.colfaxarea.com/biking.php and the entry for "Bicycling from Alta/Baxter to Truckee/Lake Tahoe".

To the south, US-50 to Tahoe would be insane. If going that way, take Mormon Emigrant Trail from Jenkinson Lake at Pollock Pines to 88, then 89 up to Tahoe.