I had a very bad experience field growing Chinese junipers in the north-east of France, friends were successfully growing them in the ground around Nice. I gave up after 3 years of marginal growth with close to no development and some branch loss. My itoigawa are now back in culture pots. I believe they do prefer having their roots warm. I had no serious issues with procumbens juniper in the past, so it is going to depend on which type of juniper you are growing.
The maples I have in the ground do thrive and grow like weeds.
Absolutely work on the nebari before putting the tree in the ground or you will waste years down the line trying to fix or replace it.
If you wire the trunks on field-grown material you will need to check them very often as the trunk will thicken faster and that may leave ugly scars.
If you want a gnarly juniper mame/shohin from cutting, you start by wiring the cutting and giving it exaggerated curves, you wire it again the next year to give counter-curves or compress, then you can start adding a shari that will enhance the curves. If you do successive/close tight curves on the trunk, the first one needs to start at soil level otherwise you will get reverse taper when the trunk fleshes out. Kinbon had an article in Fall 2009 (if I recall) on a simplified process for pines using cable ties to compress the plant after wiring it.