generally, you are correct. the reason is, imagine you accidentally drop the connector and it hits metal on the chassis: which polarity would you want your barrel to be?
of course, you would be wise to verify that orientation by the icon on original power supply.
one other thing to be aware of is the DC voltage requirement of each device you're powering. Some devices only need 4.5V while others take 12V and sometimes upwards of 18. Make sure you can deliver the same kind of clean power your wall supplies did or you might be in for some trouble.