I think anyone using a desktop that has overheating issues is likely to be over clocking.
I’ve never cleaned the dust out of a computer as I’m extremely allergic to dust and I have something that’s been running just find for the past 10 years.
Many pre-built systems use cooling solutions that are barely adequate to start with, and poor case designs for cooling.
Dell for instance, is notorious for relying on the CPU fan to be the only fan in the system, or only putting an additional 80mm directly behind a solid panel in the front. Even in their "gaming" systems.
Dust buildup is normal, especially if someone has pets. Because of your allergy you probably keep the area around your computer cleaner than most people!
In the land before time I did PC maintenance and sometimes you could remove dust from under a CPU fan in one single piece because it was matted together ... nasty, but very satisfying when you've finished cleaning and the computer just whirrs happily afterwards :-)
I think overclocking is really not necessary today, what with cpus boosting indivual cores all by themselves depending on the load and all that. Just buy a decent cooler (no need for an expensive one) and let it do its thing.
Is good to think a bit about the flow of air. Mine has more air being pushed in than pushed out so that the air doesn't enter from places where there aren't dirt filters. (In case it's not clear, what is not pushed out through fans just leaves naturally)