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 $18.90   9.474%   $20.6906   $20.69   $20.70   $20.70  
  Purchase #2 $28.65   7.614%   $30.8314   $30.83   $30.85   $30.80  
  Purchase #3 $198.70   4.406%   $207.4547   $207.45   $207.45   $207.50  
  Purchase #4 $96.94   9.663%   $106.3073   $106.31   $106.30   $106.30  
  Purchase #5 $51.48   3.397%   $53.2288   $53.23   $53.25   $53.20  
  Purchase #6 $132.82   3.400%   $137.3359   $137.34   $137.35   $137.30  
  Purchase #7 $147.40   12.319%   $165.5582   $165.56   $165.55   $165.60  
  7 purchase totals = $674.89       $721.4069   $721.41   $721.45   $721.40