Hi Rosie,
Funny you should ask that, I'm currently struggling to make part of the July Joint Effort symmetrical.
In Blocked grids (Solid black squares and solid voids or clear letter squares) it is the standard or accepted norm to make these symmetrical. In the barred grids it is very difficult to make them match with symmetry, some barred grids will allow it if quite a few shorter words are included but with longer words, it's a real challenge because of the use of every square being a letter square.
If you want to try it, get some graph paper and start off with a word with common letters and see how far you get before you're struggling to find a word to fit to enable you to carry on.
My software gives me the facility to look for words 'one at a time' which is how I do it, but as you begin to enter the corners, the choices of words available soon diminishes to less that half a dozen and even fewer sometimes. I have been working on some great puzzles in the past and got down to the last couple of word spaces and absolutely nothing will fit so I've had to undo a whole section and start on that section again. It's trial and error sometimes when you are coming to the last bit of the grid and can be so frustrating.
There is a facility to just press the 'Autofill' button and the computer software will build a grid but I do like the challenge of doing it myself. This is because I look out for new and exciting words to match with certain clue setters from the forum.
You may notice at times with the barred grids that the compiler/editor will mention in the preamble that a word is a 'made up word' that could quite easily be a word and this is usually a lengthened version of a real word that (when lengthened) doesn't actually exist but certainly looks like it could.
Can't remember what they call it now but I'm sure one of the regulars who do the barred puzzles regularly can remember it.
Hope that helps.
Les40