Laser cut tolerance is generally accepted as 0.1mm, but some cover themselves and state 0.3mm. If you specify the tag at 30.0mm, it could end up being 30.1mm. If you specifiy the slot as 30.1mm it could end up being 30.0mm (worst case), meaning its going to be hard to fit. You'll also have the same tolerance in tag position as well as tage size so if there are multiple tags these could end up being out with slots in a cumulative fashion.
Note the 0.1/0.3mm tolerance is usually stated over a certain distance so be wary of that. I'd expect a 0.1mm tolerance on a part 10mm long. A 0.1mm tolerance on a part 6000mm long might be asking too much.
Best to involve the laser cutter and get them to offer some help on what tolerance you should leave. Involves them too if there is subsequently a problem..