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 $59.57   12.322%   $66.9102   $66.91   $66.90   $66.90  
  Purchase #2 $103.48   7.171%   $110.9006   $110.90   $110.90   $110.90  
  Purchase #3 $131.29   4.831%   $137.6326   $137.63   $137.65   $137.60  
  Purchase #4 $222.60   6.740%   $237.6032   $237.60   $237.60   $237.60  
  Purchase #5 $115.80   7.129%   $124.0554   $124.06   $124.05   $124.10  
  Purchase #6 $95.34   6.662%   $101.6916   $101.69   $101.70   $101.70  
  Purchase #7 $176.76   7.769%   $190.4925   $190.49   $190.50   $190.50  
  7 purchase totals = $904.84       $969.2861   $969.28   $969.30   $969.30