You definitely do not need an attorney for every small accident case. Usually a small cases can be resolved without the assistance of an attorney.
I may no be cost effective to use an attorney for a small case because most of the recovery may go to pay the attorney and you may be left with very little afterwards.
However, the bigger the case the more likely you will benefit from attaining an experience attorney. A study by the Insurance Research Council (IRC) have shown that you will likely recover more compensation for you injuries with the assistance of a lawyer than without (even after subtracting the lawyers fee).