It's really important! It is difficult to overemphasize the importance of lot grading.
Where houses have basements, wet basements are the most common complaint. When surface water is drained away from the building, there are fewer water problems in crawlspaces and basements and much less risk of deterioration to the foundations due to hydrostatic or frost pressures. Ground sloping toward a building funnels water from rain or melting snow against the building and may cause considerable damage.
While there is disagreement about the exact slope of the grade, and how far it has to be maintained away from houses, a slope of one inch per foot for at least six feet is a common recommendation. As a rough guide, you should be looking for a slope that is visible to the eye.
Something that warrants consideration is whether or not the ground surface absorbs water or keeps water out. For example, an asphalt or concrete driveway adjacent to a building can have a slope less than one inch per foot for six feet and still be effective. As long as there is a slope down away from the house of at least one inch over four feet, that should be adequate. The other side of the coin is that an impervious surface that slopes down toward the house can be a disaster. None of the surface water will soak into an asphalt driveway, so it will all accumulate against the house.
If the ground around the building is extremely porous (sand or gravel, for example), the ground slope is not as important. Water will drain through sand or gravel very quickly rather than run across the surface. If there is sand or gravel down to a point below the footing or basement floor, water should fall past the building and not create any problems. However, if there is clay one or two feet below the sand, for example, water may get trapped at this level and find its way into the building.
Since we can't know the soil characteristics around a house all the way down to the footings, we will always recommend good slope of the surface grade.
Some lot slopes make it difficult to achieve good drainage. Swales (shallow ditches) or catch basins can be used on these difficult lots, but the potential for problems may remain significant.
There are also structural issues with respect to lot grading on significantly sloped lots. As a rough guideline, lot slopes should not exceed a one foot rise on average for every two horizontal feet. Where slopes exceed this, the risk of erosion or mud slides may be significant. This will depend on a number of local factors including soil conditions, type and amount of vegetation, amount and intensity of rainfall, etc. You should be speaking to building authorities, building associations, and builders in your area to determine what is successful in your region with respect to slope stability.