FWIW, USDA recommends against the use of wax sealing. All the rest of the world continues to practice it.
Why's the USDA against it? Because of a potential loss of seal integrity. Most people will not take the time to do it right, you see.
However, the spoilage mechanism in sugar preserves is mold. Unlike bacterial toxins, mold is highly visible. So if the food is spoiled you know it.
Personal decision point: Many people merely scrape off any mold they see and eat the rest. However, the "roots" are invisible, and could go way down in the jam or whatever, and toxins could be building up. You pays your money and takes your chances.
Here is the correct way to effect a wax seal:
1. Fill hot, sterilized jars with the hot spread, leaving 1/2 inch headspace.
2. While contents are still hot, carefully pour a 1/8th inch layer of melted paraffin. Using a needle or something similar (traditionally hatpins were used) puncture any air bubbles which appear.
3. When first layer sets up, pour in a second 1/8th inch layer of melted wax, again puncturing any airbubbles.
4. Let sealed jars sit, without moving, for at least 24 hours.
5. Move to pantry or other storage area. Do not tilt the jars while moving them, as this could cause the wax/glass seal to fail.
Compare all of this to the more modern approach, which is to process in a boiling water bath for 5 minutes.
Putting food safety issues aside, you can see how the scales balance in favor of the water bath. On one side, a dangerous, time consuming, difficult chore. On the other a simple job.