Hey, I just came across your question;
Now do you understand OpAmps?
So,as to speak the so-called instrumentation Amp?
As a matter of a fact, this is merely an extended application of the 
same Differential Input stage of 1 single OpAmp, just with several 
OpAmps conjucted for the purpose of creating a 'Groundless'
Floating Inverting (-) & Non-inverting (+) input ;-?
Dunno, where you've got the idea that such Diff.Amps Asymmetrical 
supplied
should have ?NO? Offset? on the contrary to 'standard' OpAmps?
Wich are from them selves also Diff.INstr.Amps and all have also their 
specific caracteristics, like Offset
All components inside have a certain 'matched' symmetry, but due to 
uncontrollable intrinsic wafer diffusions, NOTHING is exact the same.
In such it depends of how much you like to spend on an OpAmp for the 
purpose it is needed? You could try the T.I.'s TLC-versions?
These offer selectable offset behaviour.
In-My-Humble-Opinion, it's better to have OpAmps WITH offset and to 
compensate them for...Than to use no compensation at all...And get
surprised, with unsuspected behaviour in the long way...

Greetzs, Harry - Belgium - Europe.