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 $25.73   12.334%   $28.9035   $28.90   $28.90   $28.90  
  Purchase #2 $128.86   5.752%   $136.272   $136.27   $136.25   $136.30  
  Purchase #3 $130.41   10.357%   $143.9166   $143.92   $143.90   $143.90  
  Purchase #4 $115.55   3.174%   $119.2176   $119.22   $119.20   $119.20  
  Purchase #5 $179.54   5.614%   $189.6194   $189.62   $189.60   $189.60  
  Purchase #6 $155.62   7.398%   $167.1328   $167.13   $167.15   $167.10  
  Purchase #7 $22.54   8.755%   $24.5134   $24.51   $24.50   $24.50  
  7 purchase totals = $758.25       $809.5753   $809.57   $809.50   $809.50