Utility and consumption tax If I have a model with taxes on consumption denoted $\tau$ should I write the utility function as $u(c)$ or $u((1-\tau)c)$? Thanks
U(c). 

Taxes are an adjustment on your income/resource and show up on your budget constraint. 

U(c) just measures the utility you get from consuming c. 

As an aside, consumption taxes should not be 1-t. 

Example: Investment = F - (1+t)C = Net Savings.   

If you wrote (1-t), it would mean, Investment = F -C + tC. Higher is the tax rate, more is the investment or budget left over for investment, ceteris paribus - which does not make sense. 

EDIT

The answer by @Henry got me thinking about the following. 

If a model is proposed where a consumer purchases C units of consumption goods taxed at rate t, such that the consumer will have to give away tC units in taxes and ultimately will consume (1-t)C, the analysis is based on tangible goods. In the budget constraint of the consumer it will read as :

Investment = Output - Price * C  

since the consumer will have paid for C units and then ultimately consumed (1-t)C units due to the tax.