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 $39.98   7.581%   $43.0109   $43.01   $43.00   $43.00  
  Purchase #2 $207.98   6.559%   $221.6214   $221.62   $221.60   $221.60  
  Purchase #3 $165.95   4.263%   $173.0244   $173.02   $173.00   $173.00  
  Purchase #4 $234.48   11.102%   $260.512   $260.51   $260.50   $260.50  
  Purchase #5 $77.99   11.458%   $86.9261   $86.93   $86.95   $86.90  
  Purchase #6 $6.54   6.622%   $6.9731   $6.97   $6.95   $7.00  
  Purchase #7 $199.43   13.659%   $226.6701   $226.67   $226.65   $226.70  
  7 purchase totals = $932.35       $1,018.7380   $1,018.73   $1,018.65   $1,018.70