Yes, I was talking about for the purposes of Gematria. Actually, I was specifically wondering about substituting hebrew characters to obtain a numerical value for an english word. So the Teth/Tav thing is good to know. Appears I had that one backwards. But I suppose what matters most is internal consistency.
Thanks, Av. Ben., for your exhaustive answer. I'll keep that info in mind for when I'm doing something more in-depth than basic gematria.