This would depend on how long it was going to be in the jig.  It would also depend on how large the frame was and the type, thickness of material and the amount of area to be welded.  

Having said all of that and not knowing the specifics, I can only answer vaguely.  Most of the time, we use jigs only for the tacking of the weldment and then it is removed from the fixture and welded.  When possible, it is skip welded so that it will remain square and true throughout the welding process.  On small parts with a rather large percentage of weld area per surface area, it may be necessary to build in some slop to allow expansion and contraction of the material.  

If welding the parts in the jig, be sure to allow the parts to cool back down before removing them to avoid them moving during cooling.  It may sound like a cop-out, but trial and error is the best way to see what you can get away with on your particular part.  Hope this helps.

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