Using devices with modern Li-Ion batteries, you can charge your iPhone whenever you want / need, without restrictions.
To my experience (since my first iPhone 3g in 2009), I charge my iPhone overnight, every nights, and my batteries have always run properly for at least 2,5-3 years.
Do not worry about overcharging: when the battery reaches 100%, iphone automatically stops charging.
The only suggestion I can give you is: if possible DO NOT let the battery go below 20%.
Regards
Giulio