Theoretically,one should discount each year's cash flow at a cost of equity that matches the maturity of the cash flow.However,for practical purposes,analysts typically choose a single yield to maturity that best matches the cash flow stream being valued.

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In choosing the bond’s duration, the most theoretically sound approach is to discount each year’s cash flow at a cost of equity that matches the maturity of the cash flow. In other words, year 1 cash flows would be discounted at a cost of equity based on a one-year risk-free rate, while year 10 cash flows would be discounted at a cost of equity based on a 10-year discount rate. To do this, one should use zero-coupon bonds (known as STRIPS) rather than Treasury bonds that make interim payments. The interim payments cause their effective maturity to be much shorter than their stated maturity.
However, using multiple discount rates is quite cumbersome. Therefore, few practitioners discount each cash flow using its matched bond maturity. Instead, most choose a single yield to maturity that best matches the cash flow stream being valued.

Theoretically,one Should Discount Each Year's Cash Flow at a Cost