Currently, my workstation has two 8-port USB<->RS232 devices, one dual port USB<->RS422/RS485 adapter, and several single port adapters such as the very useful BUBII. So with around 20 USB->serial devices, figuring out which /dev/ttyUSBx entry corresponds to which port is not really practical. However, with udev in Linux, you can easily give static names to each device. This is especially convenient to do with FTDI devices because each FTDI device has a serial number. In devices such as the 8-port RS232 adapter, there are 4, 2-port FTDI chips. I could not find a serial number available in the adapter with a Prolific IC, so I would avoid those until that is sorted.
udevadm can be used to discover the serial number for a FTDI device:
udevadm info --attribute-walk -n /dev/ttyUSB0|grep serial
After the serial number is known, rules can be created in the /etc/udev/rules.d/99-usb-serial.rules as shown in this example. Now, serial ports can be accessed using convenient names such as /dev/ttyUSB_beagle.