Any regulation needed when powering an AVR MCU from USB?
If you are running the AVR at 5v, then no, no extra regulation is needed. This is of course not talking about neccesary parts like decoupling caps. Any ic should have a ~0.1uf cap near its power and ground pins.  If you are using the analog features of the AVR, also keep in mind the recommended board layout, decoupling caps and inductor.

Depending on how long your power cable is, you may want to add a 10 or 100uf cap at the input to your board.

Keep in mind that Usb allows ± 0.25v within spec. And some chargers are better than others. Some have fairly choppy/noisy regulation, so you might want check it on a scope to see how bad or good it is at keeping near 5v.

And that answer involves USB spec host behavior. USB chargers are not usb hosts. That's not an issue when using a USB charger as a simple 5v power supply.