as long as it's regulated.
don't expect an unregulated "5V" WW to work. expecially if powering any kind of digital logic. but even just leds will need fussing. this is because the unloaded voltage with be well above 5V and, with load, will drop based on the current draw. If you assume that the WW is producing 5V and then figure out your resistors based on that, you may well be way off. For example, if the WW is producing 8V and your led has a Vf of 2V then you would be doubling the current through the LED from what you expected.
an unregulated WW (often called "pulsed DC") typically has a symbol on it - a dashed line above an unbroken one. It's just a transformer, rectifier and filter cap. Regulated WWs will say "regulated" on them, it it doesn't, presume it isn't.
To run a 5V regulator, I'd make sure I have at least 7V if the load is reasonably light and 8V otherwise. That's why you see a lot of 9V WWs out there.