When using the ULN2003, it has diodes built in. All you have to do is wire pin 9 to the positive voltage rail. In most cases, when I use relays, I tend to choose 12v or 24 volt relays as they're more common and most likely cheaper. Another side effect is that the current consumption is lower. If you use 12 volt relays, pin9 of the ULN2003 goes to +12v. Also, read carefully the specs on the ULN2003 to make sure you don't exceed the load capacity of the device. I've seen a few go up in smoke due to not complying with the specs.