From my experience any pain in your sinus is serious and should be treated before diving. The main reason I would be scared is the possibility of a reverse block. Sinus pressure on the way down is a good warning to abort the dive. But if you make it all the way down and develope a reverse block you can tear your ear drum and cause serious damage with no other option than to come up before you run out of air. I’ve been there. I pushed it too early after treatment from an ear infection. It took me a while to get down, but made it and thought I was home free. Coming back up at the end of the dive I felt horrible pain, as well as periods of vertigo. I worked it slowly, a couple feet at a time, but the pain stayed with me all the way to the surface. Two days later I went to the doctor because my ear was so sore. She said my ear drum was traumatized and there was a possibility of a slight tear, and I needed to take at least two months off from diving. Waiting would have been smarter.
My first year of diving led to 4 sinus infections, each one worse than the previous. A good antibiotic should take care of the infection, and sometimes a sinus flush will help get some of the gunk out of your head aferwards. But the gunk typically works its way out by itself. I had one severe sinus infection which got worse with the first two antibiotics (I must have developed a resistance) and required a super strong "shotgun" antibiotic given in hospitals to finally reverse the infection. On the second day of that drug a giant "plug" of gunk came out during a sneeze the size of a pencil eraser. That thing was stuck in a tiny tube behind my eye?!? It certainly explained the pain.
I then discovered Doc’s Pro Plugs. They do allow water into the ear because they are vented with a small hole. But they also prevent the constant in & out flushing of bacteria infested sea water. The theory is that an ear will clear/equalize more easily in warm water than cold. The plug keeps the water inside and your head keeps that water warm. Since I started using them, almost three years now, I have not had a single ear or sinus infection. And since I don’t believe in cooincidences....I won’t dive without them.
Hope this helps.