I still CPU mine alt-coins like HBN and IFC because I have the computers on anyway.  The other scenario I have is where I'm GPU and ASIC mining bitcoin on a machine and want the CPU to do something instead of just sitting there.

There are profitability calculators out there you can use.  But don't expect to make more than pennies a day per machine even with the most profitable alt-coins if you are just cpu solo mining.