you shouldn't need one. depends on how low/tight of a cheek weld you want. a riser will raise the scope up about 3/4 of an inch so your head can be in a bit of a more upright position when shooting. it's all personal preference.

for true accuracy over a wider range of distances the closer the optic is to the bore, the better. in this case, the 15-22 is not a match target rifle so do what works best for you.