This is a bizarre question. If you don't dot the 'i', then it's not an 'i', so yes you do need to dot it or your letter will be full of spelling mistakes.
I just tried writing something and I write the word first then my hand automatically goes back and dots the i - I hadn't ever thought about it before but my brain is obviously geared to do it.
In an informal letter it is not as important to place the dot precisely, but by not bothering at all you are showing a lack of care and respect for the recipient.
Yes, lower case I has a dot so it's necessary otherwise it's not a bloody I and is a lower case L! (I don't like when people play around with letters, does it show much?!)
And I dot my I's as I go along, I couldn't move on to the next letter without the previous letter being perfect... it would give me the hee-bee-geebies.... :c)
In handwriting, no, it doesn't matter, as long as what you write is legible. But because the i is a single short stroke and gets easily lost, it's normal to make it more clearly identifiable by dotting it. Likewise a t without a cross looks like an l.