I am going to buy a bunch of solar panels to get 5v 1.5 amps for a USB phone charger. I need a precise 5v. Should I stick with the 5v solar panels or use 7v solar panels with a 5v regulator?

 

You need to regulate it.

 

And, you'll get the most power (voltage times current) from your panels when they are run at ~80% of their max voltage, not when they are drawn down by excessive current. So the panels' open-circuit voltage needs to be 5V + regulator drop out + 20% or so. Obviously, a low-dropout regulator helps.

A concurrent thread here uses a device that might help you.