I and both my dss, and my dh, could read fluently before starting school (and I do mean reading books to ourselves, reading with expression and understanding and being able to summarise what we had read). I never considered this to be a sign of a startlingly high IQ, particularly since I was the youngest of four, so a large part of my ability must have been learning through example. I did do extremely well academically, though (dh amusingly slightly less so, given that he was identified as gifted when his local state primary school advised his parents to try for scholarships at local private schools instead, resulting in him going to very good prep/public schools on full scholarships from the age of 6. He is undeniably extremely intelligent, but it takes more than that and a good education to reach your "full potential" (in quotation marks because he has actually got exactly what he wanted out of life, it just turns out what he wants is not the lofty heights of academic success...)).
However, along with reading ability, ds1, like his father, also had an uncanny ability with numbers from an early age, working out number sequences, playing with numbers, etc, despite no active encouragement of anything more than counting and no older siblings to copy. His memory and understanding of abstract concepts are also unusually good. He is not unduly inquisitive about the world around him, though - only interested in things if they can be turned into an amusing story - and this is maddening, sometimes. He also hates to be challenged, prefering things that are well within his capabilities. He has been assessed by a psychologist, at the request of the school, and has been found to be highly intelligent (amongst other things...). I don't think this is the answer to all the issues he has at school or at home, but it does affect the school's approach towards him in a positive way and was therefore helpful and worthwhile (but only because the IQ testing was part of a broader assessment of his skills and personality).
Ds1's brother, ds2, is more of an all rounder, interested in everything, less exceptional with numbers although still several years in advance of his peers, and much more talented than his brother at all things practical, as his spatial skills seem to be particularly good, whereas ds1's spatial skills are obviously his weak point. Ds2's social and emotional skills are also superior to ds1's. Ds2 is extremely happy at school and I therefore have no interest in having his IQ tested or doing anything much about it. His teachers have picked up that he is unusually bright, because in his case it is obvious and not obscured by any behavioural or physical issues, and he is mature for his age in all respects (the sort of child most teachers would want to and find easy to keep happy and engaged, though I say so myself!). Unless he ever expresses any frustration or unhappiness at school, I therefore don't think there is anything to be gained from having his IQ assessed.
So, I guess I agree with the others - you do what you feel is right for the child you have. Every child is different. Not every unusually bright or gifted child needs to be assessed to prove them as such, but some, as a result of the mix of their particular personality and IQ (and possible weaknesses - few people are gifted in all areas), may benefit from a closer assessment. But maybe wait a year or two first to see how your ds settles into school routines?