I take the bobbin case out (a top-loading type) and dust it and where it came from and as far inside that area as I can reach with a natural bristle makeup brush that hadn't been used for anything else. I find it gets into dusty places better and attracts lint better than the plastic-bristled brush that came with my machine. I also try to get the dust out of every nook and cranny I can reach above the needle. I do this between every garment unless I have two similar and fast projects back to back, like 2 knit tops. Some projects require interim dusting, such as velvet or cotton flannel. I also dust the entire outside of the machine. It's my "start a new sewing project" ritual.
I bought mine used 4 years ago and have never had it serviced. It's an entry level computer-based Viking manufactured in 1995, and I have no idea what the previous owner did for maintenance, if anything. She didn't use it much. It does not require regular oiling.
My original sewing machine, a Singer Creative Touch 1036, was the first no-oil machine that Singer made, I think. The gears under the bobbin are plastic. I got it for Christmas in 1979, my senior year of college. I only ever had that machine in for service when something no longer worked. Until I busted the upper tension module 4 years ago. I haven't had it repaired. I found this Viking on Craigs List instead.