There can be dust, liquid drops and in my case pollen issues. A quick way to check for image sensor marks is to photograph either a white wall or a white sky at the smallest aperture on the lens. Dust is more noticeable on DX sensors than FX because, whatever the size of a speck of dust, it appears proportionately 50% bigger on the smaller DX sensor. Like the D300s and D3s I have found the D7000 clean image sensor very efficient - and have it set in the set up menu to clean on start up. On a detail if there is not time to complete the cleaning cycle prior to taking the first image the cleaning cycle does not complete. There are menu options for :- clean on switch off, both - or disable. I have not had the degree of problem being illustrated with pre sensor clean bodies - but did use a "rocket blower" after each session. Keeping dust at bay (film, digital or lenses) is part good housekeeping, as in regularly cleaning equipment, bags, and avoiding changing lenses in dusty conditions - or rain Tiny drops of liquid on the sensor whilst changing lenses in rain or bad housekeeping such as breathing on the sensor to remove dust often leaves drying marks which need a wet clean to remove. Some plant pollen has a sticky surface to help it cling to surfaces - and leaves sticking marks when attempting the first clean - so do not change your lens in high pollen conditions such as walking through a meadow at the time plants are releasing pollen. With the latest sensor clean bodies I find I need to do a specific clean no more frequently than every 6 months.
Photography is a bit like archery. A technically better camera, lens or arrow may not hit the target as often as it could if the photographer or archer does not practice enough.