Most combustion occurs in gas phase. When a liquid burns, the flame zone is actually a little above the surface and the heat of the flame vapourises the fuel, which rises and mixes with air in the flame to then combust.
Bear in mind that states matter for enthalpy calculations so writing an equation for (say) ethanol liquid combusting has taken account of the ethanol vapourising.
I guess the answer is then "it depends on how the question is written / what you are writing"... sometimes it is strongly implied which to use, sometimes it's a matter of judgement, sometimes either is reasonable.
