Use a spirit level to check your overall base and ensure that it is as level as possible to assist with ease of installation.
If the base isn't level, you may find that you struggle assembling the building as the panels might not line up. Alternatively you may find that the doors and windows stick in their frames if they are not sitting square. The doors may also look as though they have dropped or are not hung correctly.
This is fairly easy to resolve and can usually be rectified by simply lifting two opposite corners of the building to straighten the building out.