The only time an electrical half wavelength multiple lengths of coax have to come into play is using older models of antenna analyzers. Many newer designs have the ability to calibrate out such things, so in that case length doesn't matter so much. Also, such coax lengths are only good for one frequency in said band, and moving away from that the results will be more and more off.
They actually still sell such antenna analyzers, such as the venerable MFJ-259 and 269. I would simply stay away from such devices as you can often get more capable devices for about the same price, or even cheaper (look up the nano-vna for example).
If your radio and or meter can't measure anything more than SWR then you don't have to worry about it as SWR doesn't change with coax length (short of losses in said coax). If you do notice your SWR changing with coax length, then you have another issue, like the common mode currents talked about above, or perhaps a ground loop. The easiest method of checking if you have such a problem is measure SWR, add a three foot jumper and measure again. If there is a noticeable difference, you have a problem.
Using such lengths for jumpers sounds at first like a good idea, but in reality, such things can actually hide other problems. I've seen cases where people would be "bitten" sometimes touching the radio, or even through the plastic housing of their microphone. I've seen other cases where the final stage transistors in radios, or the power transistors in amplifiers are blowing consistently. The cause of such problems were literally hidden by half wavelength multiples of coax.
There is actually a second use for coax length of electrical half wavelength multiples, that is your using off impedance feed-line aka 75 or 100 ohm coax on a 50 ohm radio setup. Although there really isn't a need for this in this day and age and I haven't heard of anyone needing something like this in forever.
The DB
