Although I am not necessarily one who is an avid fan of rebalancing (as I like to let winners run), it is nice to understand what one owns in a proportional sense. If a position has run too much or not enough, having a view into one's portfolio allows one to add/subtract as needed to accomplish individual goals.
To tackle this, I use some python code to scrape current stock prices from the web, and then multiply price times shares owned to obtain ownership stake in a dollar amount. Then simply divide ownership stake by total portfolio value to obtain the concentration a particular position holds within one's portfolio. For example, below illustrates how my E*Trade portfolio, which holds my Roth IRA, is structured:
Per above, I tend to trust my financial future to GOOGL, AMZN, and VUG (a low cost EFT which contains mostly big Technology names).