On the topic of Pitch Paradigms from mother tongues interfering with tones and prosody / natural sounding rhythms when speaking Thai, I've just put this visual representation together of how many learners of Thai - or just visitors to Thailand pronounce สวัสดี sawat dee (khrap /kha) -
In English when greeting someone in a friendly manner, there's a tendency to say:
'Hiiiiii, How are Youuuuuu?'
It's as though the muscles subconsciously lock into this 'friendly greeting' pattern regardless of the language. This superimposed on Thai has a big effect on the natural sound of the Thai delivery and while sure, people will understand what you're saying, it will sound very 'foreign'.
In fact, I often hear Thais especially around the Sukhumvit area of Bangkok who deal with tourists to 'take the piss' saying sawatdee in a 'Farang' way to unwitting tourists - akin to seeing an Asian and saying 'Hurrroooo'.
Is it necessary to pronounce it with the correct tones / rhythm? Up to you - but if you're looking to develop any fluency in the language, it's a good thing to be aware of and work at eliminating.