What should the input voltage be for a L7805CV voltage regulator?
Look at chapter 5 under line regulation. 

For the 5V regulator at \$V_{IN} = 7 V\$, then you will typically see a \$3 mV\$ ripple with varying load, up to \$50 mV\$ worst case. At \$V_{IN} = 8 V\$ this reduces to \$1 mV\$ typical and \$25 mV\$ worst case.

So you'll need at least \$2V\$ head room, but \$3V\$ has less ripple (which is better)

Another parameter to look at is Dropout Voltage, which is just little below the line regulation. The dropout voltage is the voltage required for the regulator (between output and input) to be able to do its job properly.  In the same table is stated typically \$V_{d} = 2V\$. But if you want to design a robust circuit, you should always use the worst case which is here \$V_{d,max} = 2.5 V\$ value to ensure the circuit will work with all regulators.