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 $35.25   10.743%   $39.0369   $39.04   $39.05   $39.00  
  Purchase #2 $66.71   6.198%   $70.8447   $70.84   $70.85   $70.80  
  Purchase #3 $208.62   9.270%   $227.9591   $227.96   $227.95   $228.00  
  Purchase #4 $44.69   7.129%   $47.876   $47.88   $47.90   $47.90  
  Purchase #5 $170.33   11.768%   $190.3744   $190.37   $190.35   $190.40  
  Purchase #6 $67.59   8.736%   $73.4947   $73.49   $73.50   $73.50  
  Purchase #7 $67.49   13.764%   $76.7793   $76.78   $76.80   $76.80  
  7 purchase totals = $660.68       $726.3651   $726.36   $726.40   $726.40