I don't buy the theory that pitching rate significantly affects flavour. The whole topic is full of pseudoscientific explanations, such as that underpitching causes stress (actually the opposite is true).
Anyway here's a 2008 paper suggesting that pitching rate makes very little difference to flavour, except for diacetyl level rising with high pitching rates (http://www.ncbi.nlm.nih.gov/pubmed/19018524
Low pitching rate is completely fine in my experience. The only problem besides waiting a day or so longer is that there's higher risk of infection before the wort is colonized by yeast.
Abstract (my emphasis):
The volumetric productivity of the beer fermentation process can be increased by using a higher pitching rate (i.e. higher inoculum size). However, the impact of the pitching rate on crucial fermentation and beer quality parameters has never been assessed systematically. In this study, five pitching rates were applied to lab-scale fermentations to investigate its impact on the yeast physiology and beer quality. The fermentation rate increased significantly and the net yeast growth was lowered with increasing pitching rate, without affecting significantly the viability and the vitality of the yeast population. The build-up of unsaturated fatty acids in the initial phase of the fermentation was repressed when higher yeast concentrations were pitched. The expression levels of the genes HSP104 and HSP12 and the concentration of trehalose were higher with increased pitching rates, suggesting a moderate exposure to stress in case of higher cell concentrations. The influence of pitching rate on aroma compound production was rather limited, with the exception of total diacetyl levels, which strongly increased with the pitching rate. These results demonstrate that most aspects of the yeast physiology and flavour balance are not significantly or negatively affected when the pitching rate is changed. However, further research is needed to fully optimise the conditions for brewing beer with high cell density populations.
A more recent paper (http://onlinelibrary.wiley.com/doi/10.1002/jib.242/abstract
With increasing numbers of cells introduced into the wort, the content of the esters and fusel alcohols increased, while the acetaldehyde concentration decreased. These changes affected the final quality of the beer.
Which contradicts the popular homebrewers' notion that underpitching makes yeast express esters more strongly because they're "stressed".
Obviously I've cherry picked these quotes to support my argument, but the best anyone can really say about pitching rate & perceptible flavour is that there's no clear scientific evidence of a relationship.