For the Red Shirt for instance I made sure to have very wide seams all ironed open, and left them that way, as the whole garment is finished off by lining.

Using a zig-zag to lock something delicate like this silk would have degraded the material, overlocking it professionally would have left unseemly marks when ironing the seams, and doing it by hand with such a number of pieces would have taken days to accomplish.

So seeing as this was a non-washable piece from the start, leaving the seams unlocked was always the most appropriate thing to do.

On a regular machine you can just as well use a wide zig-zag program to lock your seams, making sure the needle catches the edge of the fabric with every other stitch.
The result (example shown here) works just as good as an overlock, especially on a heavier kind of fabric.

The only reason I’d consider getting an overlock-machine would be for the really fine fabrics, like veils and silks, where the tension of the zig-zag thread is too strong for the lightweight material, making it gather or run in places if done this way. So these, I’m afraid, are currently still being locked by hand. ^_^;

So, locking the later types would be a bit pointless, as it takes up a lot of time and won’t really help your end-result.

In confection it is standard to leave the seams so short they have to be locked together to save them from ripping, but in general it looks a lot more professional to have 1 to 1.5 cm seam allowance left-in and have it properly ironed; open or both sides the same way.