When I first started my SM army, I went with "battleforce" approach, using a variety of units. Overall, I think that worked out fairly well, as it gave me a good idea of the strengths and weaknesses of the codex and its units before I focused in on one particular build. That and it gave me a lot of options when it came to building lists for different points levels, events, and whims. So I am going to stick to that strategy while building the GKs.
Having said that, when it comes to designing lists, I believe in:
1. Redundancy-If my lists needs to do something, then I'm not leaving it one unit
2. Duality-I greatly prefer units that can handle tanks or infantry as needed
3. Saturation-I want to force my opponent to make hard choices about targeting
4. Dispersion-I don't like too many eggs in one basket
5. Synergy-I love force multipliers IF they also fullfil the previous criteria
What I do NOT believe in is SPAM. Once again, this is the product of my Space Marine upbringing where multiple units can serve similar but slightly different roles. For example, MM Landspeeders vs attack bikes and Riflemen Dreads vs Combi/Dakka Preds. Personally, I have found that by using both in the same list provides more overall flexiblity to the list while not sacrificing the main purpose of the units. Further, by mixing the list up slightly, it tends to lead towards a more visually pleasing army (ala White Dwarf picture spreads) and better soft scores. I'm not denying the effectiveness of spam and do not feel any sort of "moral" superiority for not spamming. I just feel that in many cases that making minor changes from "optimal" units can actually be beneficial in soft scores and flexibility. That and I just get bored painting the same thing...