Mass wise: Whatever value you sub in is what value your answer will be (since the multiples will cross out).
Velocity wise: As long as both v<sup>2</sup> and c<sup>2</sup> are in the same units (ie in ratios of c, or in terms of ms<sup>-1</sup>, putting them both in terms of c is the same as multiplying them by 3*10<sup>8</sup> which crosses out)