Does anybody leave a percentage of their BTC on the exchanges, or do you guys store all your BTC in your wallets, etc?
Those percentages will vary depending on what are your personal preferences and your current strategy, those which are traders probably keep the majority of their coins on an exchange in order to take advantage of any opportunity that may present itself, long term holders may not keep a single satoshi in exchanges, while other users may decide to keep some of their money there so they can make a quick transaction in the case they want to buy something and they are on the move.
You are very correct and it is practical. Just like me, I do not have to keep all my Bitcoin in self-custody that is decentralised, which is not good for my trading business but only good for the investment part. It is the HODLers who can be comfortable with the keeping of all of their coins in the self-custody arrangement,t but for those who want to trade and will use Bitcoin for payment and also send money home to be exchanged with P2P and others, it is unwise and not necessary to keep it all in the self-custody. The way I see it is that once money is moved to the non-custodial wallets, that money should be locked there, and God forbids that the person has a problem, the money should be there until Bitcoin has a long-term bearish pattern on the monthly chart. Nothing other than that should cause the moving out of the money for any reason. But for business to continue to happen, the centralized exchange which is custodial must be used and it is even better than the DEX alternative for business in all ramifications.