One good way of getting a better layout value is to put 2 caps in series and 2 in parallel for getting more accurate layouts. For example, if you have to realize a 100 fF cap, which is the most basic value, you can put two 100 fF caps in series to get an effective cap of 50 fF and then parallel them with a similar configuration. So, effectively you get 100fF.
The drawbacks for this is more area. Hence this technique can be used for smaller values of caps. The same can be done for the resistors. If you want to realize a 10 Ohm resistor, lay 10 resistors of 100 ohm each in parallel.
If you are laying out bigger values, there is no need for this technique......