You should try experimenting with them. Pro always use /bm instead of the normal F1 - F9 hotkeys since SinX has a lot of skills and it is good to have some of skills & equipments on your hotkey in case if you need to change accordingly to the situation.
1. Comfort-ability, The easier it is, the better.
2. Memorable, It shouldn't be too complicated or out of reach from your fingers. Like out of nowhere, your yggdrasil berry at F9 ( WTF?!?! )
3. Practice, Practice, Practice. Sometimes it helps to train your hands and fingers in getting use to, to some keys. Skill/Ygg berry spammers.
Mine was like (before)
F1 - SB/SD| F2 - MA| F3 - Back Slide| Ygg Berries | Cloak | Enchant Poison | Envenom | EDP
You really don't have to follow this though lol