Disconnecting electronics, and electrical grounds as much as possible is a good idea. However, the most important thing when welding on any machine is that you be sure that your welding ground makes good contact with bare metal close to the weld on the part you are welding. You want to be sure that the circuit completed by the welding rod or wire goes directly to the ground clamp and back to the welder without going through any other part of the machine first, or to any other ground.
You do not want there to be any other path to ground. Rubber tires are good insulators from ground if they are clean and dry. The kick stand needs to be insulated from the ground by placing a non conductor between it and the ground like a block of dry wood, rubber, plastic. Do not leave any tools or metal leaning against the bike that may provide a path to ground. Never weld on anything that is wet. These secondary grounds will not carry welding current, but they can carry stray voltage and current that can damage sensitive electronics or parts.
Be sure that there are no isolators, rubber bushings, or even bolted connections between the part you are welding and the ground clamp. This is to insure that the current goes directly to the ground clamp through the direct route rather than through a whole bunch of other parts in a round about path to get there.
Connect the ground to the major part that you are welding something onto, like the frame, instead of the loose part that you are connecting to it, like a bracket. Then be sure to start your arc on that major part that is grounded and move to the loose part that is not, rather than the other way around.
If you are welding something onto the frame, then connect your ground directly to the frame close to the weld. You probably need to grind a bare spot to get rid of paint so you can insure good contact. Don't connect the ground on one end of the bike and weld on the other end. If you are welding a tab onto a fender, grind a bare spot on the fender close to the weld, and connect your ground there, rather than connecting it to the frame and making the current go through the fender connections to the frame before it can get to the ground. Whatever part you are welding something onto should be where you grind bare and connect your ground.
These procedures insure that stray voltage and arcs don't occur in undesirable places. Failure to follow these procedures can short out electronics, and destroy bearings, or other parts that have tight clearances by arcing across the gaps.
Welding on an engine requires even more care to prevent damaging the internals. The Rebel does not have any computer ECU or other fancy electronics, but if it did, you would need to take further precautions to isolate it from welding currents, since it only takes a tiny amount of current to fry them.
Hope this helps you and others avoid welding current damage.
1999 Honda Rebel CMX250C
2003 Harley Sportster 883 Hugger
1998 Harley FXDWG Dyna Wide Glide
ALL IN! GO TIGERS!