Modern electronics will not overcharge battery, and phone should be able to handle being constantly plugged in. However keeping battery constantly in high state of charge and charge cycling in high voltage will increase battery degradation. It's best for battery when phone is minimal time in full charge.
My personal experience is that my android phone SGS1 handled overnight charging all well so it's not wise to get too worried about this, but if your battery never last long keep eye on that.
Some portable devices sit in a charge cradle in the on position. The current drawn through the device is called the parasitic load and can distort the charge cycle. Battery manufacturers advise against parasitic load while charging because it induces mini-cycles, but this cannot always be avoided; a laptop connected to the AC main is such a case. The battery is being charged to 4.20V/cell and then discharged by the device. The stress level on the battery is high because the cycles occur at the 4.20V/cell threshold.
Chargers made for operational readiness, or standby mode, often let the battery voltage drop to 4.00V/cell and recharge to only 4.05V/cell instead of the full 4.20V/cell. This reduces voltage-related stress and prolongs battery life.
To minimize stress, keep the lithium-ion battery at the 4.20V/cell peak voltage as short a time as possible.
Most Li-ions are charged to 4.20V/cell and every reduction of 0.10V/cell is said to double cycle life.