No, it shouldn't affect your local console. As far as the kernel is concerned, having a terminal running on ttyS? is no different to having one run on tty?. If you press <CTRL>+<ALT>+<Fn>, you'll find that you're already running at least 4 (normally 6, and then 7 is used for X) virtual terminals.
I think that it is potentially a good idea... it lets you have a 'hell, it doesn't work!' terminal to hand. I think there's a good HOWTO on using a Psion as a serial console on TLDP
... and the principles should be the same.
Hope this helps.