Its both a yes and no. The important thing is that the laser level accuracy is good enough for the job that you are using it for.
In reality with laser level accuracy pretty much all lasers on the market today have a tolerance rated between 1mm to 6mm in 20m. For indoor work over short ranges any rating in this band would be fine as most of the time you would not be able to or need to work to any finer tolerances.
For long range outdoor lasers, something within the band of 1mm to 4mm in 20m would be fine as users are likely to generate more error variance by the way receivers and staffs they are used than by the tolerance of the laser itself. In specialised high accuracy engineering environment the tolerance of a laser becomes far more important.
The next points to cover; are the tolerance rating itself and whether the rating quoted has been checked post manufacture in the country of distribution. RedBack Lasers calibrate each and every laser level sold and then put a calibration certificate with date on the unit, how long the laser maintains this accuracy depends on the quality of build and how it is used and stored by the user. RedBack Lasers are build to the highest standards and so under normal operating circumstances calibration will rarely drift by much. RedBack lasers calibrate lasers in Australia and make them well within their quoted tolerances allowing some margin for any minor drift and still remain within tolerance. Some other brands rely on the factory calibration before shipping from outside Australia and may also select a rating based more on marketing edge rather than reality this I would suggest is quite prevalent on internet auction sites.
Most brands rate accuracy as a number millimeter over a number of meters such as 2mm over 20m or 3mm over 30m. One brand I’m aware of quotes accuracy in arc seconds which for the average person makes it impossible to compare. The reality for the brand in question is that when you convert their arc second figure over to mm per m its about the same as other comparable brands.
I hope this answers some questions on accuracy ratings on laser levels.