The answer to this question is – it depends upon the geometry/design of the part, the nut being welded and safety. M6 weld nuts come in many shapes and sizes.
Assorted M6 Weld Nuts
Workpieces also vary in shape and size. Clearance for the electrodes will be determined by the geometry of the parts involved and the fixtures and tooling being used. There is no set value for spacing other than clearance to load the parts safely and not too much clearance to make the travel time too long for the electrodes to close.
After part design and clearance are determined what is left is safety and time. Time considerations are determined by the welding and loading equipment capability and the plant investment in tooling, automation and other factors.
Safety of personnel operating the equipment is a factor that must be considered in the design and layout of the equipment. Various safety codes could apply to the operation and determine pinch point, light curtain, lock out or other safety needs. A starting point to determine these needs could be ISO 12100 Safety of machinery — General principles for design — Risk assessment and risk reduction.
Automated or manual nut welding operations all require evaluations for safety and part clearance to determine the proper electrode opening. No two operations are alike therefor there is not a set answer to this question.
For economics and safety one would not open any more than is necessary to load the part. Safety must be built in to keep the operators hands out of the pinch points. This reduces electrode travel time and reduces room for an operator to be hurt.
Reference: ISO 12100 Safety of machinery — General principles for design — Risk assessment and risk reduction.