Properly installed new cabinets should be ready for countertop installation, without any additional steps.
Older cabinets that have had the countertops removed may need additional preparation. With the countertops off the cabinets, now is a good time to check the condition of the cabinets. The cabinets must be secure to the wall and level, both front to back and level the length of the cabinets. Shim and refasten to wall as needed.
Now is also a good time to check and repair drawer slides and repair any water damage that may be around sink cabinet.
The tops of the cabinets must be free of any adhesive, nails, or screws that held down the previous countertop.
If the previous countertop was secured through corner blocks in the cabinets, and you decide to install the new countertops the same way, check to see that the corner blocks are solid. If they are not solid, reinforce them. The countertop build up strips must lay flat on the cabinets.
Once the cabinets are level and secure you can continue your installation.