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 $21.44   6.280%   $22.7864   $22.79   $22.80   $22.80  
  Purchase #2 $138.93   3.817%   $144.233   $144.23   $144.25   $144.20  
  Purchase #3 $243.33   10.477%   $268.8237   $268.82   $268.80   $268.80  
  Purchase #4 $112.33   9.411%   $122.9014   $122.90   $122.90   $122.90  
  Purchase #5 $107.62   9.149%   $117.4662   $117.47   $117.45   $117.50  
  Purchase #6 $134.53   7.673%   $144.8525   $144.85   $144.85   $144.90  
  Purchase #7 $6.70   9.558%   $7.3404   $7.34   $7.35   $7.30  
  7 purchase totals = $764.88       $828.4036   $828.40   $828.40   $828.40