There are two main schools of thought in the brewing world about the effect of bittering hops. Some believe that the type of hops added for bittering will contribute some flavour characteristics other than bitterness such as earthiness or fruitiness... the flip side is that hops added at the start of boil only contribute to general bitterness.
I personally tend to agree with the first school of thought and to explain my reasoning, let's have a look at the basic scientific principles behind the bittering process...
So the bitterness comes from the lupulin in the centre of the hop flower. When the lupulin encounters heat (ie. the boil), this heat catalyses the isomerisation of alpha acids, which means that the previously insoluble alpha acid becomes a water soluble compound. The longer the hops are boiled, the more bittering compounds are released into the wort. Meanwhile, the volatile aromatic compounds present in the hops are rapidly evaporating out of the wort. Hence, if we want to add aroma, we add hops later in the whirlpool or fermentation vessel...
That's a very simple description of the chemistry going on in the kettle, however it illustrates that bittering hops is not contributing any of its aromatic characteristics because they have long since boiled off earlier in the process and then the remaining insoluble material from the hops is removed as part of the hot break after the whirlpool stage and as such doesn't impact flavour.
Perhaps if your whirlpool is inefficient at removing the bulk of trub, you could have some flavour contribution from the spent hops sitting in the fermenter, but I doubt that is a contribution that many brewers want! If this happens in your brewing system and you're insistent on using say a 2-4% alpha acid Saaz hop flower to bitter your IPA, you may have an "earthy" flavour from the large quantity of organic matter in tank... not to mention a lot of mess to clean up later on... but really, if you've ever had the misfortune of putting a hop pellet or flower into your mouth, the bitter taste is completely overwhelming... I'd be very impressed if someone could distinguish different hop plants by taste without smelling them! What other flavour (and I'm talking about taste, not aroma) is there in the hop?
We recently planted 150 or so hops in Kyneton and I specifically planted some Super Alpha rhizomes to take advantage of the high alpha acid content of these hops to reduce materials consumption and waste in the brewing process, ie. less hops required = less mass into and out of the system, because as a production company we want to minimise our impact on the environment.
So my advice to any budding home brewers is to take the economical and efficient option by using a locally grown high alpha acid content hops for bittering purposes and save the beautiful aromatic varieties for your whirlpool and dry hop additions. Any contribution to mouth feel or flavour beyond bittering would be undetectable except to the most expertly refined palate. I have brewed the same beer recipe using different types of hops for bittering and it hasn't made a noticeable difference to the beer quality.
So that's my two cents - what do you think about bittering hops?