In most cases, shaped dough is placed into bannetons with the seam side UP, whether proofed cold or warm, and then fully inverted and scored (on the non-seam side) when moved to the oven (this is true using the combo cooker method as well in professional deck ovens).
If you have a particularly slack dough such as a ciabatta (this isn't a great example as they aren't proofed in baskets, but the point stands), you can proof seam side down in a banneton and invert before moving to the oven without slashing. In this case, the seam serves the same function as slashes (which can't typically be easily made on super-slack doughs). Another benefit of proofing a very wet/slack dough seam side down is that the tension of the unbroken top skin allows the dough itself to develop more strength while proofing. You can test this some time by shaping two rolls and placing them both on a linen sheet (couche) covered, to proof, one seam side up and the other down. After awhile, the roll with seam up will practically flatten (depending on other factors in the dough, of course), while the seam-down will have considerably more strength.
Dough is typically never "slid" out of a banneton to keep the same side up, nor should it be flipped twice. Both of these methods can damage the loaf, ruin shaping, and in some unfortunate cases create thick folds of uncooked dough in the final bread.
Finally, in some manifestations of the cast-iron cooking instructions (especially with no-knead recipes), they typically direct you to proof in a basket seam down, then invert into the heated cast iron pot without scoring, because the deep sides of the hot pot making scoring difficult (and the no-knead loaves typically also aren't very "strong"). The combo cooker method obviates this by having you invert the dough onto the "skillet/cover" part of the cooker, which is not deep and so presents less danger of burns.
Hope this is clear. Reality is that in the bread world there's little set in stone - and master bakers take the approach of thinking about the desired characteristics in the final bread and then work backward to create a process for the mixing, fermentation, shaping, proofing, scoring, and baking that has the desired outcome. Want a more rustic look, for example? Proof seam down and invert so the seam is up and breaks on its own (or skip the banneton altogether, don't overwork the shaping and use a couche to proof). Prefer decorative cuts? Proof seam up in a banneton with a cloth (to avoid ridges), invert, brush off excess flour, then dust evenly with fresh flour, score and bake.