If you are going deliver in 1080p, work in 1080p. You can always scale down/work at half resolution, render to smaller files. But scaling up can be a real hassle if it's the last thing you do, particularly when you start working with a lot of layers and precomps nested inside precomps. There are scripts that do a good job with this, but it's so easy for things to get missed, or unaligned.
Your method is going to cost you more time than it will save in the long run.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.