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 $106.73   13.567%   $121.2101   $121.21   $121.20   $121.20  
  Purchase #2 $207.91   8.136%   $224.8256   $224.83   $224.85   $224.80  
  Purchase #3 $24.42   10.381%   $26.955   $26.96   $26.95   $27.00  
  Purchase #4 $37.40   4.459%   $39.0677   $39.07   $39.05   $39.10  
  Purchase #5 $109.36   12.116%   $122.6101   $122.61   $122.60   $122.60  
  Purchase #6 $104.41   4.130%   $108.7221   $108.72   $108.70   $108.70  
  Purchase #7 $112.85   5.773%   $119.3648   $119.36   $119.35   $119.40  
  7 purchase totals = $703.08       $762.7554   $762.76   $762.70   $762.80