Let's look at the terms in this expression:

[HOCl] = x

[OH^-] = 1 x 10^-7 M + x
Here 1 x 10^-7 M represents the hydroxide concentration due to the autoionization of water and the variable x represents the extent of the ionization reaction. Since the ionization reaction will produce hydroxide ions, the autoionization equilibrium will be shifted, producing even less hydroxide than 1 x 10^-7 M. It is very likely that hydroxide concentration due to the ionization of OCl^- will be much greater than the hydroxide concentration due to the autoionization of water.

[OCl^-] = 0.71 M - x
The variable x represents the extent of ionization reaction. Since K_b is very small, it is very likely that x will be much smaller than 0.71 M.

If there are terms you think will be negligible, it is safe to ignore them as long as you go back and check that assumption after you finish the problem. If you don't check your assumptions, you may end up making invalid assumptions!