I have just built some raised garden boxes and I’m wondering if I can put potting soil in them now and leave it over the winter or leave them empty until spring.
Congratulations on building your own raised garden beds! You can certainly fill them with soil now, so that you are ready to plant in the spring. Assuming you are going to use them for vegetable gardening, if you are planning a crop of garlic, fall is the time to plant for a harvest next summer. Garlic for growing is available at many farmers’ markets this time of year. If the forecast stays warm and sunny, there is also just about enough time to plant a cool weather crop of lettuces or spinach which should give you a final harvest of tender baby leaves before the first frost.
You refer to potting soil: a combination of triple mix and compost works best in raised beds, rather than topsoil. This mix provides a good combination of nutrients in a growing medium that is light and easily worked.
Here are a couple of links to soil options for raised beds:
https://eartheasy.com/blog/2014/04/3-useful-soil-mixes-for-planters-and-raised-beds/
https://growingrealfood.com/how-to-make-garden-soil-for-raised-beds/
In the spring you may find that a few weeds emerging, but given the friable nature of the soil in your raised beds, you will be able to pull these easily by hand. One of the great advantages of raised beds, in addition to being easier on the back and knees, is that they offer protection from soil compaction and are less prone to erosion. Some gardeners cover their raised beds in winter to prevent wind erosion and subsequent loss of nutrients, either by using plastic or by a layer of leaves or mulch. Every spring you should amend your soil by adding organic material such as compost and manure to increase soil nutrients.
If you are interested in doing more reading, local author Tara Nolan’s book, Raised Bed Revolution: Build It, Fill It, Plant It… Garden Anywhere (Cool Springs Press, May 2016), is really terrific.