You can use any compatible iPad charger to charge any iPhone. You do not want to do the reverse, however (use an iPhone charger to charge an iPad), because the wattage is not high enough to efficiently charge the iPad. But an iPad charger's wattage is higher than what is required by the iPhone. The iPhone will only draw as much as it needs, so you can use it to charge the iPhone.
I have my iPad on my side table, and it is usually plugged in. However, I regularly use its cable to charge both my iPhone 4 and iPhone 5s.
Cheers,
GB