I'm not sure having multiple kinds is a must, having things to chew is a must, but different kinds of wood is just nice. I know my guys seem to enjoy getting different kinds of wood stick to chew. You may want to shop around a bit for wood chew though, although $7 for 10 oz is cheap $18 for 3oz is expensive. I give my guys about half a dozen sticks between the two of them everyday. I guess only one a day might be ok so long as they have plenty of other chew toys to chew on in the cage.
Chew toys should also be changed out regularly to keep things interesting. You don't have to throw the toys away if they are still good, but have a rotation like you would a dog, so once a month or so put some new toys in and take the old ones out, once another month rolls around you can put the old toys back in and they are "new" again. Sometimes even taking a toy out and flipping it or taking it apart and putting it back together differently can make it new again too.