I guess it depends on the inks and paper you use. I use the Magic Mix inks and MagicJet paper and don't give washing instructions. I have washed them in about every detergent there is. My wife is a bargain hunter and gets the largest bottle for the best price. LOL.
God Bless You
If you are selling transfer printed t-shirts to customers, then you also have to consider that customers don't really care which detergent they "should" use, most likely they will use whatever brand they normally use.
If you tell them they should use a specific detergent, that could be a red flag to them that your product may not be of the highest quality or may not stand up to "normal" washing.
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