[Fzang]

If

A + B -> C

And C is a solid, eg. a salt which is highly soluble in water and the reaction above is happening in aqueous solution, should C be noted as (aq) or (s)?

[Fluorine]

Aqueous, unless I misunderstood your question as being simpler than it is. If it has high solubility in water then it won't form a precipitate and couldn't be regarded as a solid. Granted my answer is disregarding temperature, volume of water, moles of product, and possible exceptions I was never taught.

HCl_(aq) + NaOH_(aq) NaCl_(aq)

Yet NaCl itself is a solid when not in water.

[skbuncks]

As Fluorine noted, you would only mark it as (s) if the product precipitated out of solution.

HCl_(aq) + NaOH_(aq) NaCl_(aq)

Yet NaCl itself is a solid when not in water.

...as is NaOH. Just saying 

skb

[Fzang]

Thanks guys