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   5.477%   $58.9933   $58.99   $59.00   $59.00  
  Purchase #2 $115.49   4.147%   $120.2794   $120.28   $120.30   $120.30  
  Purchase #3 $7.98   8.687%   $8.6732   $8.67   $8.65   $8.70  
  Purchase #4 $246.57   12.187%   $276.6195   $276.62   $276.60   $276.60  
  Purchase #5 $70.60   3.214%   $72.8691   $72.87   $72.85   $72.90  
  Purchase #6 $226.89   5.870%   $240.2084   $240.21   $240.20   $240.20  
  Purchase #7 $156.88   3.222%   $161.9347   $161.93   $161.95   $161.90  
  7 purchase totals = $880.34       $939.5776   $939.57   $939.55   $939.60