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 $240.78   10.791%   $266.7626   $266.76   $266.75   $266.80  
  Purchase #2 $55.69   12.527%   $62.6663   $62.67   $62.65   $62.70  
  Purchase #3 $23.40   4.558%   $24.4666   $24.47   $24.45   $24.50  
  Purchase #4 $29.42   10.407%   $32.4817   $32.48   $32.50   $32.50  
  Purchase #5 $125.28   10.262%   $138.1362   $138.14   $138.15   $138.10  
  Purchase #6 $45.45   10.681%   $50.3045   $50.30   $50.30   $50.30  
  Purchase #7 $165.39   9.623%   $181.3055   $181.31   $181.30   $181.30  
  7 purchase totals = $685.41       $756.1234   $756.13   $756.10   $756.20