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 $7.91   12.280%   $8.8813   $8.88   $8.90   $8.90  
  Purchase #2 $170.64   11.466%   $190.2056   $190.21   $190.20   $190.20  
  Purchase #3 $101.75   13.632%   $115.6206   $115.62   $115.60   $115.60  
  Purchase #4 $146.51   5.864%   $155.1013   $155.10   $155.10   $155.10  
  Purchase #5 $36.78   10.730%   $40.7265   $40.73   $40.75   $40.70  
  Purchase #6 $68.81   13.257%   $77.9321   $77.93   $77.95   $77.90  
  Purchase #7 $170.33   5.673%   $179.9928   $179.99   $180.00   $180.00  
  7 purchase totals = $702.73       $768.4602   $768.46   $768.50   $768.40