Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $249.95   13.493%   $283.6758   $283.68   $283.70   $283.70  
  Purchase #2 $204.95   13.421%   $232.4563   $232.46   $232.45   $232.50  
  Purchase #3 $19.28   10.246%   $21.2554   $21.26   $21.25   $21.30  
  Purchase #4 $198.69   12.308%   $223.1448   $223.14   $223.15   $223.10  
  Purchase #5 $26.55   13.422%   $30.1135   $30.11   $30.10   $30.10  
  Purchase #6 $212.86   11.725%   $237.8178   $237.82   $237.80   $237.80  
  Purchase #7 $110.70   11.277%   $123.1836   $123.18   $123.20   $123.20  
  7 purchase totals = $1,022.98       $1,151.6472   $1,151.65   $1,151.65   $1,151.70