-->Colour gurus such as Dan Margulis and Michael Kieran recommend a "trip" to the Lab mode Are they wrong?
In almost all cases, yes, certainly with 8-bit files (Dan doesn’t believe in anything more). You lose a heck of a lot of tones, depending on the original gamma of the working space. The question becomes whether it’s worth taking the time or worse, producing image degradation to convert from a working space to LAB and back. Every time a conversion to LAB is produced, the rounding errors and severe gamut mismatch between the two spaces can account for data loss, known as quantization errors. The amount of data loss depends on the original gamut size and gamma of the working space. For example, if the working space is Adobe RGB, which has 256 values available, converting to 8-bit LAB reduces the data down to 234 values. The net result is a loss of 22 levels. Doing the same conversions from ProPhoto RGB reduces the data to only 225 values, producing a loss of 31 levels.
One advantage of LAB is that since the colors (the A*axis and B*axis) are separate from the luminance (L*axis), it is possible to conduct tonal corrections that do not affect color. Hue shifts are avoided when changing lightness.
By and large, LAB editing is over-rated and with the tools in Photoshop, not often necessary.