We do both -- leave the charge on our account but also hand over the envelope in person.
As PP mentions, you get "coupons" to put in the envelopes if you charge the tips to your onboard account. I believe most people give those envelopes directly to the CM, sometimes adding extra cash. If you have charged tips to your account, the CMs will receive it whether or not you give them the envelope with coupon. However, it's helpful for them to make sure they do in fact receive the correct payment, and also a nice time to say "thank you" in person.
If you decide to remove the charges from your onboard account, the only way to give tips is by handing over the envelope.
Enjoy your cruise!