To your first question no. To your second question: The tax returns of all cantons are based on Swiss law and not the law of the canton. The canton can set different things that can be offset and vary the amounts of the offsets etc. It can further decide for itself how to interpret your tax return but that is about it so a treuhand from one canton can help you complete a tax return for another canton only it might not be fully optimised. Oh I forget to add. ***Generally***, if the tax return has been completed by a treuhand then the tax authorities will simply accept it as they are going on the principle that it has been reviewed and completed by someone who knows what they are doing. If the treuhand is from another canton they are very likely to inspect it closely to make sure they are not filling in something according to "their" cantons rules.