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 $17.81   6.357%   $18.9422   $18.94   $18.95   $18.90  
  Purchase #2 $104.92   6.545%   $111.787   $111.79   $111.80   $111.80  
  Purchase #3 $121.64   11.412%   $135.5216   $135.52   $135.50   $135.50  
  Purchase #4 $200.41   8.757%   $217.9599   $217.96   $217.95   $218.00  
  Purchase #5 $149.97   5.583%   $158.3428   $158.34   $158.35   $158.30  
  Purchase #6 $143.96   9.784%   $158.045   $158.05   $158.05   $158.00  
  Purchase #7 $40.30   3.435%   $41.6843   $41.68   $41.70   $41.70  
  7 purchase totals = $779.01       $842.2828   $842.28   $842.30   $842.20