I use paintable latex acrylic with good results. No need to paint, but you can if you want to.

thanks!

Is there any need to caulk walls not facing the outside if your primary goal is to minimize heat loss (these don't really have much of a gap if any).

These may be stupid questions since the only thing I have ever caulked are bath tubs and storm windows ....

Is the best way to get a smooth junction to use a wet finger ? Should I consider using painters tape ?

You can also use clear acrylic caulk to fill the gap.

I prefer having all the walls in a bathroom caulked to prevent any water from getting into the structure, like when a toilet overflows.

For a joint that large I would stuff a foam backer rod or something compressible into the joint so the sealant will form against it, be thin and more flexible and seal well to the top and bottom rather than just ooze back into the joint and separate at the edges as the joint moves (I would assume a lot of movement for a joint that large).

foam backer rod

Yep! (Somehow or other, I was thinking a quarter centimeter. That's what you get for being born in Imperial working in Canada's mixed system!)

any need to caulk walls not facing the outside if your primary goal is to minimize heat loss

Yes. Many of these walls connect to joist and wall spaces connected to poorly sealed uninsulated rims, basement and attic spaces. Check in any vanity cabinets around drains and supply pipes. I use expanding foam around the drains.

best way to get a smooth junction to use a wet finger

You quickly run out of clean fingers. (And it dries out your hands.) The pros I've had doing this carry a bucket of warm water and a bunch of rags with them and have learned to be so stingy with the caulking that there's no mess.

I've seen waterproofing pros who didn't need to touch the sealant after placing it. I believe it helps to place the right size backer rods carefully, use electric guns, and keep the sealant at the right temperature. They considered spreading the sealant to a tapered edge to compromise the seal. I've been able to do it for 10 ft or more but then I get a cramp or lose my balance.

It may be worth the extra couple of minutes to paint, once the caulk has set up. I've found that dust/hair/etc "sticks" to caulk rather differently than painted baseboards - having them both with the same finish makes it easier to clean.

Thanks all for the comments and suggestions. Looks like I have a weekend project coming up! I think I will plan on painting, which means I may use painters tape to caulk, and paint (unless that sounds crazy). For some reason we get a lot of dust on our second floor (not sure why since we had our ducts recently cleaned) and I'd prefer that not to stick to the caulk.

I agree with the painting the caulk, and backerrod where necessary, I would suggest a more flexible latex paintable caulk for external walls because in my house the temp. variation out to in caused the cheaper stuff to produce cracks. For interior walls, and for floor board cracks I used the reg. paintable caulk...works fine.

I'm doing the exact same project in my 100 year old home. I've stuffed thin strips of insulation material (or just left over rope) in the larger gaps and then caulked over them. I'm not a very good caulker, so it is slow painful work. After a few days I tape and paint. Three rooms down and three to go. It looks so much better than having the uneven gap!

I've done a lot of caulking baseboards!
I use a clear caulk that goes on white and dries clear.
shows you where you stopped if you get back to work before
it dries.
I keep a wet cloth with me to clean the caulk off my fingers, and rinse it often.
Of course I'm caulking to stop air infiltration, not so much for looks. Still a neat job is the goal, I've never used painters tape.
If you are stopping air infiltration you seal the top of the baseboard to the wall, and the bottom to the floor.
If you don't want the caulk to show..remove the baseboard
and caulk then reinstall the baseboard.
I also caulk ceiling moldings at the top to the ceiling and bottom to the wall. as with floor moldings you can
remove the molding and caulk then reinstall the moldings.
Dap brand 50 year caulk.
Don't buy cheap as it doesn't last, and I wouldn't want to
have to do it twice!
You would be amazed at how much air leakage these two areas
contribute to the whole house infiltration.
best of luck.