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 $6.31   9.153%   $6.8876   $6.89   $6.90   $6.90  
  Purchase #2 $78.51   9.787%   $86.1938   $86.19   $86.20   $86.20  
  Purchase #3 $247.71   13.382%   $280.8586   $280.86   $280.85   $280.90  
  Purchase #4 $113.25   7.254%   $121.4652   $121.47   $121.45   $121.50  
  Purchase #5 $86.49   5.500%   $91.247   $91.25   $91.25   $91.20  
  Purchase #6 $37.70   8.835%   $41.0308   $41.03   $41.05   $41.00  
  Purchase #7 $101.78   6.760%   $108.6603   $108.66   $108.65   $108.70  
  7 purchase totals = $671.75       $736.3433   $736.35   $736.35   $736.40