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 $108.88   12.511%   $122.502   $122.50   $122.50   $122.50  
  Purchase #2 $71.79   11.387%   $79.9647   $79.96   $79.95   $80.00  
  Purchase #3 $98.74   6.349%   $105.009   $105.01   $105.00   $105.00  
  Purchase #4 $50.86   8.852%   $55.3621   $55.36   $55.35   $55.40  
  Purchase #5 $44.80   13.384%   $50.796   $50.80   $50.80   $50.80  
  Purchase #6 $222.79   5.230%   $234.4419   $234.44   $234.45   $234.40  
  Purchase #7 $53.62   11.777%   $59.9348   $59.93   $59.95   $59.90  
  7 purchase totals = $651.48       $708.0105   $708.00   $708.00   $708.00