Let's just start with the question: Do guinea pigs need salt licks? The answer: No. It's best to skip the salt licks (or mineral) and save your piggie bank for healthier offerings. Unlimited grass hay is the most important part of a guinea pig's diet. At least 80% of what she consumes should be hay. Adding a nutritionally complete pellet and a variety of greens is all she needs for a balanced menu.
You probably realized guinea pigs are ALWAYS hungry... the first time you tried to grab a midnight snack... from the fridge... for yourself. Once they know the sound of the refrigerator opening, you're committed. There's nothing wrong with offering a healthy treat, but it's definitely up to us to make sure anything we feed is guinea pig-safe.
Guinea pigs don't need more than .5g/kg of sodium, which they easily get from pellets alone. Feeding a variety of fresh vegetables every day will also ensure they're getting a good balance of nutrients, no salt lick required.
Why Too Much Salt is Harmful
Guinea pigs aren't able to sweat like we do to get rid of excess sodium. Biologically they really have no use for extra salt either. Offering a salt lick just puts extra stress on their little bodies. A dietary imbalance of calcium and phosphorus can also cause serious bladder issues. Extra minerals, coupled with the potential for dehydration from too much salt puts your guinea pig at risk for bladder sludge or stones. Guinea pigs, like rabbits, are prone to forming urinary tract calculi. These painful uroliths can be located anywhere in the urinary tract and are usually composed of calcium salts. Once they've formed, surgery is almost always required to remove them (especially in males with the longer, narrow urethra) and they are notorious for returning... so it's better to avoid in the first place.
A Little Won't Hurt, Right?
Maybe yes, maybe no. Many guinea pigs enjoy the taste of salt, just like we do, and will go to town on a salt lick. However, this doesn't mean they should. Some lucky humans don't seem to suffer immediate ill effects from too much junk food, but that doesn't make it a good idea. Guinea pigs aren't able to make these decisions for themselves, so it's up to us to offer only safe food and treats. Providing a healthy treat (like a sprig of cilantro or slice of fruit) or a safe chew will be just as appreciated as a potentially risky item.
A Healthy Supplement to Consider
While guinea pigs don't need salt licks, it's worth keeping an eye on their vitamin C intake. Guinea pigs have a unique need for vitamin C because they aren't able to create it in their bodies. Luckily, a high-quality pellet with stabilized vitamin C in combination with feeding a variety of vegetables meets the ~25 mg requirement sufficient for most guinea pigs. Some other great options to supplement vitamin C in your guinea pig's diet? Bell peppers are one. Drops are another.
So. While guinea pigs absolutely need a diet specialized for their needs, a salt (or mineral) lick isn't needed. Actually, it's best to totally stay away from them. Lean on the good stuff to provide your cavy the nutrients needed to keep 'em in tip-top shape.