I would keep the speaker cable lengths as close to equal as possible. As it is under the floor, you could add some hooks to keep it off the floor- but, don't coil it as you might introduce some electromagnetic anomalies.
Much depends upon your cable and it's design. If it is something like zip cord, then you will probably have issues. If it something using more advanced design, say shielding and dielectric boxes, the results might be more subtle.
Bob