Welding Q......

I don't need to but have always wondered .
What would happen if you welded a steel  or alloy frame without taking any parts off ?  Ok ,maybe take off the ecu . I have heard of people stripping the whole bike ,seems excessive .They weld cars intact (don't they ?)

There are so many "earths" on a frame ( modern bike), the chance of current traveling thru and getting to an electrical component would not be worth the risk.  JMO.

Of course I'm talking elec' welding, not oxy.

Frames are stripped as part of prep work. 
Grinding out the crack isn't fun if you can't move the frame around at odd angles. Quite often welding is required on the inside as well.
As we know electrical current will travel across the shortest distance. So assuming the earth clamp was closer to the job than the ECU it should be fine. However if it was mine I'd be unhooking it. We used to have to isolate batteries for safety's sake.

Fully qualified steel fabricator here with over 10 years on the workshop floor and laying in mud under dozens and other equipment.  I'm half deaf to prove it. A lot of that time was waving a TIG torch on SS & Ally.

Yep, as Chris Said. Take the ECU off. It's the only bit of equipment that's sensitive enough to shit itself.