Pretty much any blocked low is generally negative enough to be low grabbed with a few exceptions, since i5 2T's are guaranteed. Whereas blocked standing strikes can be a little iffy and hence riskier.
@Force_of_Nature: Yeah the only safe lows I can think of are the low jabs for each character and pretty much every low kick is unsafe. But I was actually surprised by the fact that many people low throw punish but they don't punish standing unsafe strikes. As if they only know how to throw punish low strikes.