The standard assumes mains transients are attenuated by power transformers, so they'll still exist on the secondary side but much smaller:
6.7.3 Insulation for secondary circuits derived from MAINS CIRCUITS of OVERVOLTAGE CATEGORY II up to 300V
6.7.3.1 General
"In this standard, secondary circuits are circuits where separation from MAINS CIRCUITS is achieved by a transformer in which the primary windings are separated from the secondary windings by REINFORCED INSULATION, DOUBLE INSULATION, or a screen connected to the PROTECTIVE CONDUCTOR TERMINAL.
NOTE These circuits are assumed to be subjected to lower TRANSIENT OVERVOLTAGE levels than the MAINS CIRCUIT."
For your product, you must also know the Pollution Degree and what method of protection against electrical shock - grounded secondary, double-insulation, reinforced insulation etc. you are going with.
On the mains primary-side, you'll still need to meet 61010 creepage/clearances from likely Table 4.
Careful your transformer/SMPS/plugpack meets 61010, or you will have to get it tested- which is a very expensive surprise and not worth it.
On the secondary side, use Clearances Table 6, knowing the mains voltage and OVC incoming to the power supply, and the secondary voltage.
i.e. with 150-300VRMS mains, OVC II incoming and a 24VAC secondary use 33VAC for 0.52mm clearance. Creepage is Table 7, I'd use 25V or 32VRMS PDII for 0.04mm on the PC board.