Therefore, I have personally chosen to never again target anything lower than version 3.2 with instanced arrays and shader objects. Having said that, supporting OpenGL 2.x and OpenGL >3.1 at the same time is a nightmare, because there are non-trivial changes in the shading language which go far beyond #define in varying and which will bite you regularly.
If you ever intend to sell your application, you have to ask yourself whether someone who cannot afford (or does not want to afford) this would be someone you'd reasonably expect to pay for your software. The market price for an entry level OpenGL 3.3 compatible card with roughly 1000x the processing power of a high end OpenGL 1.4 card was around $25 some two years ago. Targetting OpenGL 2.1 is reasonable, there are hardly any systems nowadays which don't support that (even assuming a minimum of OpenGL 3.2 may be an entirely reasonable choice). There is little to gain (apart from a support nightmare) from that. Which means that if someone has OpenGL 1.4 and can run your shaders, he is using 8-10 year old drivers. Since using "shaders that are core in 3.0" necessarily means that the graphics card must be capable of at least some version of GLSL, this rules out any hardware that is not capable of providing at least OpenGL 2.0.
Unless you really have to support 10 year old graphics cards for some reason, I strongly recommend targetting OpenGL 2.0 instead of 1.4 (in fact, I'd even go as far as targetting version 2.1).
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