Clean printed receipts tell a customer that the POS machine has had its thermal print heads regularly maintained and cleaned. It is easy for dirt, dust, oil, pollen or grease to get inside thermal printers, and if they do, the platen rollers or print head is then contaminated.
However, preventative regular cleaning can lengthen a printer’s life.
The results of a bad thermal print job – such as a faded or streaked receipt, or a label with a poor barcode – means that the print will be hard to read. Such issues are associated with dirty print heads.

A thermal printer produces an image by selectively heating coated thermal paper when the paper passes over the thermal print head. The heated areas cause the coating to turn black.
The materials used by a thermal printer include paper, a platen and a rubber roller that presses paper against a sprung printhead. If platen rollers or printheads get dirty, the heat is not properly conducted to the print medium.
Simple weekly prevention includes using cleaning pens, cards, snap swabs and wipes.
Each time paper is changed a cleaning card will be sent through the printer, while cleaning pens need access to the print head. Cleaning clothes wipe over print heads and platen rollers, while snap swabs can be used on stubborn areas.
Source: http://www.cleanmo.com/stickymat/index.php/the-importance-of-cleaning-thermal-printheads/