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 $18.42   10.718%   $20.3943   $20.39   $20.40   $20.40  
  Purchase #2 $217.68   4.402%   $227.2623   $227.26   $227.25   $227.30  
  Purchase #3 $142.50   10.272%   $157.1376   $157.14   $157.15   $157.10  
  Purchase #4 $226.65   6.287%   $240.8995   $240.90   $240.90   $240.90  
  Purchase #5 $55.33   11.745%   $61.8285   $61.83   $61.85   $61.80  
  Purchase #6 $224.36   5.331%   $236.3206   $236.32   $236.30   $236.30  
  Purchase #7 $77.96   12.707%   $87.8664   $87.87   $87.85   $87.90  
  7 purchase totals = $962.90       $1,031.7092   $1,031.71   $1,031.70   $1,031.70