Agreed - if you catch it early enough freshly drawn wax can be straightened out into the frame very easily, but if they get a bit carried away, or you don't notice it during quick checks of the colony, it can get carried away.
However I don't use foundation for the guides. Within the brood box* I tend to put empty foundationless frames between drawn comb and it gets drawn straight, usually directly over the wires etc.
My experience of supers is more mixed, because the bees will continue to deepen uncapped comb, sometimes in preference to drawing a new frame, and you can end up with some interesting shapes that only really fit in one way next to each other, are difficult to move or remove, and as soon as there's nectar in any of the comb, trying to do anything about it in the apiary becomes challenging/messy/ a bad idea. Foundationless in supers seems to work well between frames of capped honey though, and given the experience of dealing with irregularly drawn comb, my suggestion would be to split the "new" frames when adding a full box of foundationless with the capped frames in an existing super already on the hive, giving you two boxes of half-and-half interleaved. Much quicker and easier than sorting out the mess later, if slightly longer than just adding a box once the comb is drawn.
*Brood box - of course this works in a box which the bees are using to raise brood, according to the time of year, configuration of hive, etc. If the bees are storing nectar on a frame or in a box that you want them to raise brood in, all of the cell extension issues for supers are still likely to occur.
My limited experience, but it is of being foundationless (rather than 50% foundation).