I think free-flowing text is better as it leaves the reader's side to decide how they want it presented. For example when I'm reading email on my mobile, pre-wrapped text has random line breaks that make harder to read. To disable sending wrapped text emails on Thunderbird:
mailnews.wraplength = 0 mail.wrap_long_lines = false plain_text.wrap_long_lines = false
These settings require restarting Thunderbird to take effect.