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 $19.47   4.736%   $20.3921   $20.39   $20.40   $20.40  
  Purchase #2 $149.34   4.322%   $155.7945   $155.79   $155.80   $155.80  
  Purchase #3 $133.80   3.741%   $138.8055   $138.81   $138.80   $138.80  
  Purchase #4 $129.52   10.668%   $143.3372   $143.34   $143.35   $143.30  
  Purchase #5 $46.26   5.496%   $48.8024   $48.80   $48.80   $48.80  
  Purchase #6 $202.64   13.571%   $230.1403   $230.14   $230.15   $230.10  
  Purchase #7 $179.63   12.653%   $202.3586   $202.36   $202.35   $202.40  
  7 purchase totals = $860.66       $939.6306   $939.63   $939.65   $939.60