Why Do You Ask For A NAICS Code Instead Of The SIC Code?
The five digit codes are used due to a lack of sufficient comps for many six digit codes. However, the five digit code will include any transactions from the six digit code referenced.
At present, we don't have any way of using 6 digit codes. The multiples for some specific industries may be somewhat higher or lower, but the level of revenues and earnings are much more influential than any differential in multiples between the 5 and 6 digit codes.
The benefits of isolating the fewer transactions would be less than the benefits of a larger group of "guideline companies".
Generally accepted valuation principles and procedures allow for the use of "similar" transactions (they do not need to be identical as in the exact same industry) based on other investment characteristics besides the industry code.