Is it always accurate to add Debt to Equity Value when calculating Enterprise Value?

In most cases, yes, because the terms of a debt agreement usually say that debt must be refinanced in an acquisition. And in most cases a buyer will pay off a seller’s debt, so it is accurate to say that any debt “adds” to the purchase price. 

However, there could always be exceptions where the buyer does not pay off the debt. These are rare and I’ve personally never seen it, but once again “never say never” applies.