If you are using just 5v then even a cheapo type 12v to USB unit or indeed any other type of 5v controller should not be affected by any input voltage variations providing the input voltage stays above 7.2 volts..
So unless something really odd is happening it will make no difference PW
If you are using something that specifically states 12v ( so not made for use in connection with any vehicle because all vehicle related kit is has its own built in regulator ) then the easy and cheap solution is to buy and mount an LM7812 (
http://www.hobbytronics.co.uk/voltage-regulator-l7812 ) and use that off of any feed point.
The input voltage can be anywhere from 12v to 35v and the output will remain constant..
It is easy to use.. Pin 1 goes to any feed, pin 2 to earth and pin three to your output.. They can handle an amp with ease
Although all circuits will show capacitors added on your application whilst being nice, would not really be essential mate
Whole deal, less than 50p