on the PC? like i just unplug the mp3 player, but the USB cable is still attach on the PC. is it ok to just leave it there? cuz every time i wanna recharge my mp3, i have to crouch and find the USB behind the PC. it's kinda annoying.

yes it is 100% okay.
i've been doing that for years. the most damage you can receive is having the USB cable plugged in to something (your MP3, etc).

but if it's not plugged into anything, there is no damage as technically nothing is connected to the USB port.

i would say yes. i do the same thing here with 2 USB devices on occasions.....one being my digital camera.

turn the device off first then unplug the USB cable from the device and hang that end of the cable somewhere conveniantly close (here i have 4 small nails lol hammered into the edge of the desk on which i hang the ends when not in use)

.
Central Coast NSW Aussie

There are 10 types of people in this world: those who understand binary and those who don't.

I think it is OK. I did like what you do, and nothing bad happened.

Shop for HP Laptop Battery at sunvalleyus.com. You'll find low, disount prices on replacement HP Laptop Batteries. Our HP Laptop Batteries are specifically designed for your HP laptop models.
click

With no device connected to the end of the USB cable the USB port sees an open circuit, which is exactly the same thing it sees if no cable is connected. It's just like an extension cord coming from a wall socket. Just keep the cable in an out-of-the-way place so you don't accidently zap it with static.

Sorry, you meant the cable.
Might as well leave it in. More likely you'd damage the connector before you'd damage it with esd. Only a guess though.

"Best Practices", Event viewer, host file, perfmon, are in my top 10