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 $27.40   11.703%   $30.6066   $30.61   $30.60   $30.60  
  Purchase #2 $150.33   5.771%   $159.0055   $159.01   $159.00   $159.00  
  Purchase #3 $244.85   9.785%   $268.8086   $268.81   $268.80   $268.80  
  Purchase #4 $26.56   8.693%   $28.8689   $28.87   $28.85   $28.90  
  Purchase #5 $67.64   5.305%   $71.2283   $71.23   $71.25   $71.20  
  Purchase #6 $229.71   7.290%   $246.4559   $246.46   $246.45   $246.50  
  Purchase #7 $44.70   11.354%   $49.7752   $49.78   $49.80   $49.80  
  7 purchase totals = $791.19       $854.7490   $854.77   $854.75   $854.80