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 $55.93   6.488%   $59.5587   $59.56   $59.55   $59.60  
  Purchase #2 $104.48   11.252%   $116.2361   $116.24   $116.25   $116.20  
  Purchase #3 $9.45   4.338%   $9.8599   $9.86   $9.85   $9.90  
  Purchase #4 $246.43   12.239%   $276.5906   $276.59   $276.60   $276.60  
  Purchase #5 $95.25   9.398%   $104.2016   $104.20   $104.20   $104.20  
  Purchase #6 $75.38   11.366%   $83.9477   $83.95   $83.95   $83.90  
  Purchase #7 $166.87   3.551%   $172.7956   $172.80   $172.80   $172.80  
  7 purchase totals = $753.79       $823.1902   $823.20   $823.20   $823.20