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 $25.40   10.158%   $27.9801   $27.98   $28.00   $28.00  
  Purchase #2 $26.71   11.650%   $29.8217   $29.82   $29.80   $29.80  
  Purchase #3 $134.92   8.310%   $146.1319   $146.13   $146.15   $146.10  
  Purchase #4 $65.68   5.777%   $69.4743   $69.47   $69.45   $69.50  
  Purchase #5 $214.66   6.680%   $228.9993   $229.00   $229.00   $229.00  
  Purchase #6 $205.33   9.725%   $225.2983   $225.30   $225.30   $225.30  
  Purchase #7 $229.64   9.600%   $251.6854   $251.69   $251.70   $251.70  
  7 purchase totals = $902.34       $979.3910   $979.39   $979.40   $979.40