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 $201.46   8.729%   $219.0454   $219.05   $219.05   $219.00  
  Purchase #2 $144.76   3.452%   $149.7571   $149.76   $149.75   $149.80  
  Purchase #3 $156.55   6.446%   $166.6412   $166.64   $166.65   $166.60  
  Purchase #4 $225.26   8.233%   $243.8057   $243.81   $243.80   $243.80  
  Purchase #5 $58.65   11.710%   $65.5179   $65.52   $65.50   $65.50  
  Purchase #6 $19.66   5.377%   $20.7171   $20.72   $20.70   $20.70  
  Purchase #7 $50.68   4.679%   $53.0513   $53.05   $53.05   $53.10  
  7 purchase totals = $857.02       $918.5357   $918.55   $918.50   $918.50