Is potassium metabisulfite necessary for sanitizing the fermentation bucket?

Sanitizing the fermentation bucket (and all equipment that will be in contact with the must) is recommended before making any new batch of wine, but Potassium Metabisulfite is not the only product, you can use StarSan, Aceptox, etc. It is often used when making wine since wine naturally contains sulfites and adding some more is often desired for preservation. You may wear a mask when handling the powder, but once it is mixed with water it is ok. I use a spray bottle with a mixture of water and Metabisulfite to sanitize my equipment when making wine, and Aceptox when making beer.

Most wines contain Sulfites, so the little that could be dried on the fermenter will be added to the wine, which is usually not enough to reach the recommended amount, so winemakers often add even more (camden caplets, etc.) to protect the wine from oxygen, allowing for a longer stabilization/conditioning.

The "general principle" is that the more air is allowed to be in contact with the wine, the more sulfites are depleted. Once no more sulfites are present, the wines oxydizes.

Sanitation is critical with each and every batch. Anyone can get lucky a few times. But eventually you will have problems if you don't properly sanitize your fermentation equipment.

Metabisulfite does not need to be rinsed or dried. If you are concerned about it going airborne or producing sulfurous gases, consider an alternative no-rinse sanitizer such as StarSan, which is probably even more effective, and not hazardous at all.

Cheers.