Small caulk holes around new windows

I don't think your friend's suggestion is bad, but I'd still want to be sure the caulk extended all the way to the exposed surface of the siding. Better not to let water get into the crack at all than to let it get in and stuck between siding and trim but blocked from penetrating further by caulk.

Why?

Because in winter, water that's in the crack will freeze, expand, and make bigger cracks. Water on the surface can do no harm.

There are two primary uses for caulk:

1. aesthetic gap filler
2. functional moisture/air barrier

For the latter, you'd see that wherever you need to waterproof joints (shower panels, exterior windows, etc.)

For the former, it can really be used in any situation where you have two pieces of material meet. Typically this is done when two different materials meet and you want to cover a small gap. Example would be where door trim meets the sheetrock. Higher end finishing would have that caulked before painting.

So, as for these shelves, it's really up to you. One option is to caulk it. Another would be to attach some trim moulding.

As for how to properly caulk, the basic method is:

• cut tip to size
• apply to joint
• smooth to finish

There are tools to smooth it, but I prefer to just use a wet finger (and plenty of paper towels to wipe finger off as you go...)

Note if the gap is too wide, you may need to fill the gap first. I've used foam weather stripping in the past for this.