Your best bet is to always turn on MOO right before placing down a lot from the gallery (if you want the lot to look exactly the same as it did in the gallery pics) and then turning it back off after the lot has been plunked down (if you don't want MOO on anymore).
If you don't have MOO turned on before placing a lot, any objects that MOO was used on will automatically snap over to the nearest grid (or delete it completely) when the game is placing it onto the lot, which of course, changes the way the lot looks. Sometimes that changed look is drastic (tons of stuff had MOO), and sometimes the change isn't even noticeable (MOO was only used on a couple of things).
But it's not really that big of a deal if you have MOO turned off when you place a lot. It will only effect the objects that MOO was used on. All you have to do after the lot is placed down is go have a look and see if anything looks weird or wonky, and move those things around to your liking or get rid of them altogether.
If a gallery build is giving you routing problems, then you have two choices:
1) Rearrange things on the lot until the routing problem is fixed (this could take two minutes or two hours).
2) Bulldoze the lot and download something else.
In a nutshell... Having MOO on or off isn't going to ruin/corrupt your game in any way. And having MOO on or off when placing a gallery build also isn't going to ruin/corrupt your game in any way. The MOO situation only effects the build lot itself, the rest of your game stays exactly the same.