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 $25.65   12.619%   $28.8868   $28.89   $28.90   $28.90  
  Purchase #2 $241.99   4.574%   $253.0586   $253.06   $253.05   $253.10  
  Purchase #3 $109.37   7.847%   $117.9523   $117.95   $117.95   $118.00  
  Purchase #4 $118.82   3.650%   $123.1569   $123.16   $123.15   $123.20  
  Purchase #5 $9.98   13.182%   $11.2956   $11.30   $11.30   $11.30  
  Purchase #6 $16.25   5.745%   $17.1836   $17.18   $17.20   $17.20  
  Purchase #7 $106.71   12.787%   $120.355   $120.36   $120.35   $120.40  
  7 purchase totals = $628.77       $671.8888   $671.90   $671.90   $672.10