I think Proudmama's original question,
Why not test sight words as well? is a good one.
When i asked which graphemes would be tested in the phonics check, Feenie referred me to Phase 5 of Letters and Sounds - the official UK government guidance for phonics teaching (first published in 2007).
Mrz has since given us the 64 graphemes:
a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q(u),
r, s, t, u, v, w, x, y, z ch, ck, ff, ll, ng, sh, ss,
th, zz, ar, ee, oi, oo, or, a-e, ai, ay, aw, ay, ea, e-e, er, ew, ie-e, ie, ir, oa, o-e, ou, ow, oy, ue, u-e, ur, air, igh, wh and ph* in words with the structures CVC, VCC, CVCC, CCVC.
L&S also recommends the teaching of the following 54 sight words
Phase 2: the, to, I, no, go
P 3 : he, she, we, me, be, was, my, her, you, they, all, are
P 4: said, so, have, some, come, were, there, one, do, when, what.
P 5: treasure, oh, their, people, Mr, Mrs, looked, called, asked
water, where, who, again, thought, through, work, mouse, many,
laughed, because, different, any, eye, friends, once, please.
So why not test those as well.
Being able to read the most high frequency tricky words easily is probably at least as crucial to overall reading progress as basic phonics.