Can I Only Bleed The Rear Brakes?

The other week my driver side rear brake line went out and I replaced the
line. I did a small bleed through the line because I did not want the bleeder
valve to snap off because it is so rusty. The brakes are still very spongy and
I think I have air in the line. In about 2-3 weeks I am going to replace both
rear wheel cylinders and bleed the brakes properly. My question is, I haven't
checked the front bleeder valves and I am just thinking they are rusty as
hell. I want to just replace the 2 wheel cylinders and then just bleed the back
2 brakes. Do I have to bleed the front brakes with them or not? I know it is
better to bleed all brakes, but currently do not have too much time, or
money to possibly replace the front calipers if the bleeder valves are pretty
rusty.