Since you know you have the option of taking out the I-94s, why are you asking?
You obviously need to show an I-94 when you come back, otherwise you would need to get a new TN.
Your past experience was not the common one. Besides, it appears they let you in, so such a lterr was not really necessary. Bring a paystub or your security badge.
And a copy of your i-94 also protects you if you lose the real ones, and avoids any fight at check-in.