Some breeds will need more grooming than others, however even the short coated breeds like the attention, and regular grooming is the best way to keep a close look out for any skin problems. A soft brush is best for these guinea pigs.
The longer haired breeds such as the Sheltie and Peruvian need a lot more attention, and a comb, as well as a brush, is needed in the hairdressing kit. Daily grooming is best, and Peruvians that enter shows often have their flowing locks kept in wrappers to keep then clean and tidy. Long haired guinea pigs that are kept as pets benefit from having the hair around their bottoms trimmed short, otherwise they are forever weeing down their trousers!
The Abyssinians, those are the ones with so many rosettes that their hair grows in many different directions, are groomed most easily with an old toothbrush!
It shouldn't really be necessary to bath your guinea pig, unless you want it to look extra smart before a pet show. However, all guinea pigs have a grease gland where their tail should be, and in the older ones, especially males, their coat can become covered in dark grease. This grease can be loosened with a lubricant such as swarfega, and then the whole guinea pig can be shampooed and rinsed well afterwards.