Like many things in the writing and art world there are different schools of thought on this. But, personally, I try not to take anything too seriously. What I’m trying to say is, if you want to write haiku in 5-7-5 syllable format go for it. I still do from time to time. But, I’m also on the side of people who set the record straight. The people who let the English speaking world know that your haiku does not need to be 5-7-5 syllables.
Your haiku doesn’t have to be strictly 5-7-5 syllables because haiku is from Japan and their language does not have syllables. And if you read some famous haiku you might find strange stuff like a haiku that has only one word on one of the three lines. One word? That’s not 5-7-5 no matter how you translate it.
Instead of always sticking with 5-7-5 syllables, what’s more important in English haiku is: 1) Write about nature 2) Stick with three lines 3) You can still count syllables since it's English haiku, but don’t worry about 5-7-5. Just stick to about 10 to 17 syllables total.
Here’s more on this topic along with a handy checklist:
Good luck with your haiku. And remember, haiku isn’t a joke. It’s a serious and difficult craft. It’s not easy to stick with three lines yet make the reader feel like they’ve been transported.