Normal "100%" accuracy moves such as Ice Beam, Spore and Struggle have 0xFF as their accuracy byte, which means they have 255/256 accuracy. Moves such as Swift, Swords Dance, Haze and other moves that target the user or both fields also have 0xFF iirc, but as far as I know, the accuracy value is not checked by the game when using these moves.
The hexadecimal error apllies to everything with a "X" chance of happening. Moves that are supposed to have lets say 95% acc for example, have a slightly smaller accuracy as well: 242/256. Ice Beam's freeze ratio isn't exactly 10%, but 26/255. Critical hit ratios are multiplied by 255/256 as well. Etc.
If you want, I guess, you can test Swift yourself in a rby rom by changing it's accuracy byte to 0x00, and you'll see how it still doesn't miss. You can do this with an attack editor or by hand with an hex editor (attacks info starts at offset 0x38000).