I have a question about replacing caulk. People always state that when you replace caulk, that you should remove all of it. My tub has caulking in red found in the image above. Do I really have to remove -all- of it or merely the parts that have mold (the parts at the bottom)
"Remove all of it" means entirely remove every trace of old caulk from the areas that you are recaulking. Don't put new caulk on top of old. Establish smooth clean surfaces for the new caulk.
It does not mean that you have to keep expanding the scope of your project until you reach the end of the caulk line in every direction. It is ok to leave caulk that's in good condition. You just have to decide where to put the seams between old and new caulk lines, and don't create lots of them.
In the case of a bath tub in an alcove (like most are) I would redo the entire upper rim of the tub where it meets the wall, all three sides, in one continuous bead, and if necessary also a few inches up the walls where they meet in the corners. After that I'd evaluate what's necessary. You don't need to go all the way up the wall-corners or to do the ceiling-to-wall joints if they are in good shape. It's ok to have a few joints between old and new caulk, just better not to do it where water pools.