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 $178.31   6.449%   $189.8092   $189.81   $189.80   $189.80  
  Purchase #2 $27.70   12.609%   $31.1927   $31.19   $31.20   $31.20  
  Purchase #3 $96.62   8.470%   $104.8037   $104.80   $104.80   $104.80  
  Purchase #4 $69.76   12.513%   $78.4891   $78.49   $78.50   $78.50  
  Purchase #5 $210.93   5.249%   $222.0017   $222.00   $222.00   $222.00  
  Purchase #6 $152.90   5.403%   $161.1612   $161.16   $161.15   $161.20  
  Purchase #7 $50.45   9.356%   $55.1701   $55.17   $55.15   $55.20  
  7 purchase totals = $786.67       $842.6277   $842.62   $842.60   $842.70