You need to be careful if using an unregulated supply.
A regulated supply should put out pretty close to it's rated output voltage, but an unregulated supply typically puts out it's rated voltage only when loaded to a significant portion of it's rated current. At lower currents the voltage tends to rise often reaching 1.5x the rated output voltage.
For example, the common 6V "wall wart" often puts out nearly 9V when lightly loaded.
3V and 5V are common "regulation required" values, so supplies rated 3 or 5v are more likely to be regulated, but I'd measure to be certain. I have seen 4V wall-warts which were not regulated.
Dave