It is inappropriate for most people to use such measures like op income - maintenance capex or Enterprise value. Op income does not include interest expense, which is a real expense. It is an expense which is here to stay unless you can change the capital structure of the company. It is not easily done, and it is only appropriate to look at if you're actually planning to alter the capital structure via a buyout or influencing the board of directors.
Taxes are a real expense too. Companies have in the past been able to re-incorporate themselves in countries in lower tax rates, or have international operations which they can use to leverage the dilution of taxes.
For everyday investors like the most of us, who do not have the power to enact such changes, it's best not use measures which leave out specific expenses. If you do, then you're making a bet that someone in the near future will attempt such restructuring for you, and that they are also successful. If this does not work out as planned, then the margin of safety would be reduced by using a more aggressive valuation. Attempting to predict such events is beyond the scope of how I invest, but it would indeed be appropriate analysis for a firm intending to acquire / influence a target in such a manner.