I used to mix wire sizes often, sometimes because code requires it!
On very long runs, I would upgrade wire size to try to avoid voltage drop.
When Sharing a neutral I would upgrade the neutral.
When I had more than 3 circuits in one conduit, I had to upgrade.
And even in residential, I would often run all of my home runs to the panel in #12.
When you feel that you have a good reason for doing so, the inspectors I dealt with always let a larger than required wire size pass.
On a new home, seeing #12 and #14 mingled and mixed would probably get any inspectors attention. If he feels that you have no pride in your craft, he may decide to go over your job with a fine tooth comb. To avoid problems, make your jobs as neat as you can