Can you over charge your radio and rec batteries?

Yes
Overcharging will damage your batteries.
Some chargers have peak detection and will shut down or go into trickle charge when it detects peak voltage.
If you don't have a peak detection charger you need to calculate how long to charge your batteries.
I figure I use close to 100 mah per flight on my receiver batteries and about 50 mah on my transmitter.
A wall charger supplies about 50ma charge so I charge my reciever battery 2 hours for every flight I had that day and my transmitter battery 1 hour per flight.
Do a little math based on the charge rate on your charger and you will be ok. An extra hour or two of charge won't hurt but don't leave it on for extended amount of time at a full charge rate.

While I don't recommend it, I do have a friend who keeps his radios on charge whenever they are at home 24x7. He has never experienced ANY problems or capacity losses in several years of doing this.
Myself, I prefer to charge overnight the night before I am going to fly. Period. I also have never had a battery issue in about 30 years of flying.
There are many theories/practices that seem to work.

Both of us use the factory wall-warts in these practices, no fast chargers.

G'day Mate,
Check out this site, it will answer all your charging questions.
http://members.aol.com/davthacker/balancecharging.htm