If I add another Cisco switch to an existing switch?

Dot1Q is by far more common and recommended these days. Do you have multiple VLAN's on the two switches in concern here? If not, then no need to trunk. Also, make sure that they are both VTP Transparent (preferable) or at least in the same domain (less preferrable). If you are using VTP in any non-transparent setting, I suggest rebooting the NEW switch just before being added so as to ensure no VTP/VLAN issues. Hope this helps.