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I've always been taught to ground radios straight to the battery on their own. And run a fuse on that negative lead should something try to seek a ground through the rig.
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If the negative lead in the radio is floating (not connected to the chassis of the radio or the shield of the coax) then that is fine.
Most modern ham mobile units have the negative lead electrically connected to the radio's chassis and coax shield. With a direct to battery connection for the negative lead, this allows other currents to flow through the radio seeking battery negative. Electricity seeks the path of least resistance and your direct to battery connection is mighty tempting. Also with a corroded or loose engine/chassis connection to battery negative/ground, this would be significantly worse. During starting many hundreds of amps could seek a path through the radio and cause the negative fuse to blow.
Although this would prevent a fire, the momentary high current could damage the radio before the fuse could blow. Small traces on a PC board could be damaged before the fuse could do its job. Smaller currents from other electrical devices/accessories in the car seeking a better ground could cause noise.
Note* never run an unfused negative lead directly to battery in any accessory you add to your car because there is nothing to prevent lots of amps seeking negative from burning that wire up. Starters pulling hundreds of amps could easily start a fire in the case of a poor engine ground to battery.
It is much better to ground the radio, with no fuse, to the nearest chassis ground so there is no direct negative path through the radio for high current starter loads in the case of a poor battery to engine connection. Any other smaller currents that would seek a ground through the radio would not see a better path than they have already since there is no direct to the battery connection.
The best answer is do not connect to the battery negative post unless the device has a floating ground buss in it and a negative fuse. Even then there is no reason to go to battery but it is ok if that is what is desired. If it is a more modern device with common negative/ground to radio chassis then the ground lead should be connected to the nearest vehicle chassis ground possible and there is no reason to be fused.
The reason people say that going to the battery negative prevents noise is due to poor bonding from the veh body to chassis. A ground strap or two added to the vehicle would alleviate that.