I think it is fine to slice and dice your portfolio when it is possible to diversify that specifically; however, many people do not have some of those asset classes available in a 401k plan. For simplicity's sake, a 50-25-25 breakdown is going to get you most of way.
Ibbotson has done some quantitative analysis of various portfolios and they give the following asset allocation for an aggressive growth porfolio:
48% S&P 500
22% MSCI EAFE Index (international developed world)
11% Domestic small-caps
10% Intermediate-term bonds
5% Emerging market
4% REIT
So that is about a 50-10-25-10-5 mix (Large cap-Small cap-International-Bonds-REIT).
See
Aggressive Growth ETF Asset Allocation Portfolio - Holdings