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Blending is not a simple issue. A few different factors go into whether or not it will be necessary. A premium paint system with a large number of variant shades, supported by a sprayed-paint shade deck gives a huge advantage in paint selection and will reduce the number of paint blends necessary very significantly.
Not many repairers will make the necessary investment though. There are ongoing costs with shade deck updates as well as the higher capital cost of the initial system and more expensive paint refills.
There is a myth that VM bodyshops just 'order in' the paint which will be the same batch used at the factory and therefore always obtain perfect matches. Completely untrue. The VM will specify the mixing scheme that must be used in repairs to retain the approval, but the paint will still be hand-mixed on site (or in dodgy cases, bought in from a factors (supposedly) supplying the right paint - but in this case it will not be selected by variant shade). Again, cutting corners increases the likelihood of a blend.
There is the further complication that for effect paints with a high particulate content, application affects final perceived colour and tone. You could take the exact same batch of silver paint that was used on a factory line, and if you sprayed it differently (with different technique, film build, spray equipment or pressure, drying method) or in a different environment (electrostatics, ambient temperature, panel temperature, humidity) then the final look would vary perceptibly from the factory finish.
Because of the way effect paints sit on the panel, it is often necessary to apply a drop coat or mist coat of paint for particulate control, the entire width of the panel (or at least to a blend zone, marked by an angle change or curve on the bodywork) to avoid a perceptible edge too.
Generally, the paler the colour, and the higher the particulate content, the more likely it will be that a colour will need to be blended wide to harmonise with the original finish. For a dark colour, flat, or with a small amount of mica or metallic, a decent paint system should mean blending beyond the damaged panel isn't necessary.
All the variables mean there can't be a hard and fast rule. A repairer will make a judgment call when viewing the actual piece of damage on the actual damaged panel.
Issues of match become even more entertaining when you consider that colour perception varies from one pair of eyes to another, and also when viewed under different wavelengths of available light. Wikipedia's entries on colour perception and metamerism cover the ground pretty well.
Higher skill levels and investment in a more capable paint system will reduce the amount of painting of undamaged panels, as a general rule.
HTH
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