Caulk between the segments of this modular shower surround?
You probably don't need to, as the lap in the surround design will drain water effectively. However, those are traps for mildew and grime. 

I'd clean them as deeply as you reasonably can with rubbing alcohol, then put in a bead of 100% white silicone. If you trim your tip carefully you can inject caulk to a depth of about 1/2" for good grab. 

The tube tip will tool the caulk to some extent if you size and shape it correctly. Use caulk flow rate and movement speed to prevent squeeze-out of caulk around the tip. You should then be able to use a damp finger to apply a final cove surface without squeezing caulk out onto the shoulders of the panels. Use firm pressure and wipe your finger immediately if caulk starts to pile up under it. The result will be a nicely coved joint that's nearly identical to the factory inside corner joints.