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 $123.54   3.698%   $128.1085   $128.11   $128.10   $128.10  
  Purchase #2 $66.53   6.243%   $70.6835   $70.68   $70.70   $70.70  
  Purchase #3 $74.61   9.108%   $81.4055   $81.41   $81.40   $81.40  
  Purchase #4 $204.55   11.307%   $227.6785   $227.68   $227.70   $227.70  
  Purchase #5 $198.45   13.112%   $224.4708   $224.47   $224.45   $224.50  
  Purchase #6 $25.58   11.272%   $28.4634   $28.46   $28.45   $28.50  
  Purchase #7 $220.66   3.814%   $229.076   $229.08   $229.10   $229.10  
  7 purchase totals = $913.92       $989.8862   $989.89   $989.90   $990.00