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 $173.79   8.746%   $188.9897   $188.99   $189.00   $189.00  
  Purchase #2 $133.97   6.236%   $142.3244   $142.32   $142.30   $142.30  
  Purchase #3 $52.63   10.386%   $58.0962   $58.10   $58.10   $58.10  
  Purchase #4 $5.50   8.655%   $5.976   $5.98   $6.00   $6.00  
  Purchase #5 $61.73   12.817%   $69.6419   $69.64   $69.65   $69.60  
  Purchase #6 $112.80   9.168%   $123.1415   $123.14   $123.15   $123.10  
  Purchase #7 $133.46   12.122%   $149.638   $149.64   $149.65   $149.60  
  7 purchase totals = $673.88       $737.8077   $737.81   $737.85   $737.70