it depends on your job and the what type of traveling you're doing. i recently sat next to a multi-color hair, tattooed-covered, grunge looking guy and his wife in first. it turns out this guy was the lead singer of a famous band (name withheld) returning home after an awards show. he was extremely nice and polite and i got the guy’s autograph, email and cell phone with an invitation to come down and visit when I’m in town, but the fa's treated him poorly, while the catered to me. if a t-shirt is what you're comfortable with, you can wear a jacket over it. you'll always take it off once you sit down. regardless, airline folks and hotel attendants will always look at guys in jackets more favorably than one in a t-shirt. they are cynical, tired and in no moods to be talked to. that's just the way things are. they're used to seeing thousands of people a day and you have a few seconds to impress them if you're looking for an upgrade, good seat or better room, regardless of your status, especially if you have none. i work in technology/consulting and my experience is that techies/programmers don't really care about how they dress, while consultants, sales and executives do because it’s important to feel businesslike. looking like a ceo will make you feel like a ceo, even if you're in jeans. you don't see many ceo's in t-shirts or tank tops.