design various from circuit to circuit.....there are some standard rules for cb desigers
1.if it is a high current carrying trace, should set thicker as much as u can
2.an amplifer,speaker,mic etc needs traces with impedence matching ,means that the length should match
3.for couping capcitors should place as close to the IC and trace length must set min
4. kindly follow the data sheet for some very critical component ,they will give some hint for that......and many more