I've seen both:

import scipy as sp

and:

import scipy as sc

Is there an official preference listed anywhere?

For example, in the Introduction of the Scipy documentation, it is recommended to

import numpy as np
import matplotlib as mpl
import matplotlib.pyplot as plt

but a similar abbreviation is not offered for the Scipy package.

In this question, sp is recommended, but the link to the Scipy docs doesn't actually specify sp over sc.

The "official" answer, according to the Scipy documentation, is that there is really no reason to ever

import scipy

since all of the interesting functions in Scipy are actually located in the submodules, which are not automatically imported. Therefore, the recommended method is to use

from scipy import fftpack
from scipy import integrate

then, functions can be called with

fftpack.fft()

Personally, I always use

import scipy.fftpack

and live with the slightly longer function call

scipy.fftpack.fft(data)

This way I know where the functions are coming from.