Watch the end of a band class in many schools and you'll see it - students in a hurry to get their instruments put away. Even when you’re short on time, pulling the swab through the instrument should be part of your pack-up routine. If you know you’ll be playing for an hour or more, even over the course of the day, you might go so far as to stop and swab the instrument in the middle of practicing. This will help eliminate and prevent “bubbles” under the keys (you know, those gurgling sounds you occasionally hear).
Condensation shouldn’t sit on the inside of your instrument for other reasons, too. Not only can packing a warm, wet instrument into a closed, dark case lead to bacterial growth (yuck!), but the moisture sitting on the wood or metal can cause physical damage. It doesn’t take much for wood to crack and split (think of an old wooden picnic bench), and a flute or saxophone exposed over time to slightly acidic moisture will start to oxidize and corrode (think of a wet penny left out in the open). So, don’t think you’re off the hook because you don’t have a wooden instrument!
Swab your instrument when you’re done playing. It only takes a few seconds to gently pull through, but will do much to preserve the life of your instrument.