solve inside set

All objects in HFSS have a check box for whether or not to "Solve Inside". It is usually set by default except for object of high conductivity.

For example when adding "Solve Inside" to a copper microstrip trace, a warning is issued "Solving inside a solid with high conductivity may require a large mesh".

The most important information for me to capture is the RF resistance of the copper. If "Solve Inside" is not set, will I be missing this data? or since current only flows in the skin depth, does HFSS automatically capture this?

You do not need to check "Solve Inside" to take into account ohmic losses. Just define material properties or assign "Finite Conductivity" as outer boundary condition.
Check this option only if you are really looking for a fileld structure inside a skin layer.