Exactly: I carry Windex and paper towels in the trunk for exactly this purpose. I think the thing I love most about getting a new car every couple decades is that the windshield doesn't have any pockmarks or scratches for the first few months, and I do my best to keep it that way as long as possible.
BTW, if anyone has a recommendation for glass cleaner that is better than Windex, especially for getting rid of the dim film caused by the inevitable interior outgassing, I'd love to hear them. At present, I find I have to do a couple passes with Windex and clean towels before all the smearing of that film is gone.
Others on the forum also recommend carrying bug remover, especially if you have the refreshed design model S (my car) or the model X. The new no-grill fascia accumulates a lot of dead insects over a 500 mile drive. Better to get those off the paint to avoid semi-permanent marks.