I think it just depends on what the speaker has in mind:
- No son tan sanas como parece.
They're not as healthy as it seems. -> The speaker is certain that I think they are healthy, so they state a warning that this is a wrong idea.
- No son tan sanas como pareciera.
They aren't as healthy as it may seem. -> The speaker is more cautious or may not be certain I think they're healthy.
Side note: In principle I agree with Rusty on using "parecen"/"parecieran", for the sake of the agreement with the subject. Yet, I'm keeping "parece" and "pareciera", because the speaker may have in mind the situation rather than the tortillas themselves, so it's a valid construction for me.
