At best on the the 300DC range the reading will be plus or minus 1V , possibly plus or minus 2V
so its not going to be easy to read the 5V supply with any certainty
On the other hand your meter is ideal for working on the mains electrical supply
no current range you can short circuit the mains supply with the test probes
you just need to remember not to have it on a resistance range on a live circuit
For your low voltage test like the USB supply a multimeter like this would be better
Attachment 433226
on the 20V range you measure to 2 decimal places
off load I would expect you to measure 5.1V
and a minimum of 4.75 V with a half amp load on a USB port
John