I know that in NLog you generally just do the trace at whatever level you want and it will take care of whether or not the log level should be traced or not.
To me it looks like TraceSource works the same way.
So I would say "No" you probably shouldn't check.
Test it out by setting different trace levels and tracing messages at different levels and see what gets traced.
I think in terms of performance you are generally ok if you use the methods defined on the class:
Based on an example from: http://msdn.microsoft.com/en-us/library/sdzz33s6.aspx
This is good:
ts.TraceEvent(TraceEventType.Verbose, 3, "File {0} not found.", "test");
This would be bad:
string potentialErrorMessageToDisplay = string.Format( "File {0} not found.", "test" );
ts.TraceEvent(TraceEventType.Verbose, 3, potentialErrorMessageToDisplay );
In the first case the library probably avoids the call to string.Format if the error level won't be logged anyway. In the second case, string.Format is always called.