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 $138.74   13.307%   $157.2021   $157.20   $157.20   $157.20  
  Purchase #2 $222.39   4.308%   $231.9706   $231.97   $231.95   $232.00  
  Purchase #3 $22.60   4.115%   $23.53   $23.53   $23.55   $23.50  
  Purchase #4 $217.34   4.601%   $227.3398   $227.34   $227.35   $227.30  
  Purchase #5 $181.62   13.232%   $205.652   $205.65   $205.65   $205.70  
  Purchase #6 $17.71   12.106%   $19.854   $19.85   $19.85   $19.90  
  Purchase #7 $106.63   10.435%   $117.7568   $117.76   $117.75   $117.80  
  7 purchase totals = $907.03       $983.3053   $983.30   $983.30   $983.40