ir is a requirement to show your onward fligh5 for the tourist visa when you applied online. However if you are entering Thailand without a visa for 60-days you may need to show your airline to board your flight as they might bar you.
its fine for 90 days but if you have an accident only one person speaks English (or you speak thai) so you will lose even of its not your fault. I'm really sorry to tell you this but that's how it works.