Does anybody leave a percentage of their BTC on the exchanges, or do you guys store all your BTC in your wallets, etc?
There's a popular slogan in the crypto space "Not your keys, not your coins", which simply means you're not in complete control of your coin except it's stored in your personal wallet. So I wouldn't advise you to store your Bitcoins on exchange because it could be frozen by the exchange, you could lose it if the exchange go bankrupt or get seized by the government. My advice is that you get a reputable open source software or hardware wallet and store your funds, by so doing you're in custody your asset. However, one reason you can keep some of your assets on exchange is if you're a daily trader.
Not to be harsh but the slogan itself already says it all. So there is no need for you to define it again. Also, when it comes to crypto wallets, we have two kinds here. That is custodial and non-custodial. So simply saying to store coins in a personal wallet compared to the exchanges may still not be valid. Better to be clear. OP says a percentage of BTC only.
The first thing that came in to my mind is these are only small amounts, which are not life-threatening even when lost. There maybe a reason on why someone will do that. It could be that they can earn an incentive for doing so. But it was the traders are mostly the ones who can do that.