Well, speaking from the viewpoint of currently having 4 dogs and 1 foster...
Our oldest two, Bogey and Corky (see sig block), Bogey has travelled with us for most trips except for in the last 3-4 years. He's going to be 16 and has significant arthritis so although he likes to travel, we don't take him anywhere that is more than 3 hours one way. He'll now stay home with our son or will have to be boarded when my son leaves for college next month. Corky hates the car...she shakes and trembles so she's always been a stay-at-home dog except for the occassional 1-3 hour trip.
Our two youngest, Deni, Ally, and my current foster,are travelling fiends. They've (Deni & Ally) already been to CA (22 hours one way) and will be going camping in CO/visiting relatives in WY next week with us. Cutie Pie, my tiny Cockapoo foster, will be taking her first long trip when I bring her to a no-kill rescue in CO next week. They go with me into town if the weather isn't hot and they take turns going with me to the shelter that I volunteer at and hang out with the staff when I'm taking Petfinder pictures. They will be my primary travelling partners from now on, and it helps that they are all small (< or just over 5# each). I haven't tried to sneak them in anywhere, but when I do travel I look for places that will let pets in so that they can do things with me.
There have been a couple of times that I've taken all four dogs out of town and into pet friendly stores by myself. It can be a little crazy at times, but everyone (almost) enjoys the time out. A trip to the vet with all four, by myself, can be interesting...