SWR is not an issue at all, but common mode currents can be an issue. Common mode currents are caused by improper balance of the feedlines. In other words if you fed a balanced or not perfectly UNbalanced antenna from the coax without a good balun you could have current flowing down the outside of the coax shield, and that could couple. It really has nothing to do with SWR which is a parameter INSIDE the coax.
In a practical application we run about 35 feet of CAT5 next to transmission lines in my contesting barn. The spacing is several inches or less. I don't give it a second's thought about interaction even running 1500 watts because the field from the antennas several hundred feet away is much stronger than any cable radiation from the feeders. But of course my feedpoints are all well-designed and the cables are all properly grounded at the entrance.
If you do have problems they can be fixed easily with a toroid core or two.
I have a 2" diameter #67 mix toroid with several turns of the CAT5 positioned on each end on the computer cables. I felt it totally unnecessary to use shielded CAT-5 since any common mode currents are largely unaffected by the shield. It is impossible to shield for the effects of common mode, and common mode will always be the primary issue.
You can see my wiring layout at:http://www.w8ji.com/contest_station_w8ji.htm
The blue wires are the CAT cables. We use some CAT5 and CAT3 for antenna switching cables also.