Of course, it is best to have a temperature and humidity controlled environment for any electronics assembly environment. Ideally, the temperature would be in the range of 70-77 F and the relative humidity would remain in the range of 35-65%. The question really comes down to a cost-benefit analysis based on the facts in your particular case.
Lack of humidity and temperature control can influence defect levels in wave soldering and SMT applications. Of these two processes, the one that is more susceptible to added defect levels is probably the SMT process, based on the materials and components that are used in this area. Lack of temperature and humidity control can have the following effects on the SMT process (among others):
Low humidity: Solder paste solvent evaporates quicker than expected, causing the paste to dry out. This creates poor release from stencils and insufficient solder joint defects.
Low temperature: Paste viscosity increases, creating poor printing behavior (rolling, release, etc.). This can cause insufficient solder joint defects.
High humidity: Solder paste accepts water and may begin to slump, creating additional bridging defects. Solder paste accepts water and may exhibit poor coalescence, resulting in solderballing defects. Moisture-sensitive components stored in this area will have shorter shelf life, possibly creating defects and/or damage to the components during processing. Excess water absorbed by the entire system (boards, components, paste) can out-gas during reflow and increase the size and incidence of voiding underneath BGA components.
High temperature: Solder paste viscosity decreases, possibly creating an excess of paste smearing, slumping, etc. This can cause bridging or solderballing defects. Higher temperatures in the factory induce extra oxidation of solder, boards and components prior to soldering, meaning that solderability can be compromised with higher factory temperatures.
It's worth noting that some solder pastes have wider operating windows than others. If controlling the environment is not an option, selecting a more forgiving material may help improve soldering results.
Given all of this, the company needs to determine the cost of upgrading the temperature and humidity control versus the costs that could be saved by reducing the defect levels. Also, what amount of additional business would your company have a better chance of capturing within the EMS market if your factory exhibited better environmental control? In most cases, I advise companies to take control over the environment rather than operate in extreme temperature and humidity conditions -- this is certainly the best strategy from a defect reduction standpoint. However, the decision in your company would still need to be analyzed from a cost-benefit approach given your defect levels, type of boards being assembled, customer expectations and other factors.