When we married last year (second marriages for both of us) I was too involved in launching a project to take more than a few days off. We never had a honeymoon and now we're going to take one---several weeks in the Mediterranean.
After buying a house together and joining households, we stayed with our current "S" withholding allowances. Although we could have changed our W-4's and greatly reduced our withholding, we didn't. We let the government save the money for us last year. Based on our experience with our tax returns for 2005 (we also bought points on our mortgage last year that were deductible) and the IRS worksheets, we adjusted our withholdings for 2006. Now the government isn't saving for us, but the thousands we got back this year are funding our belated honeymoon.