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 $35.70   12.639%   $40.2121   $40.21   $40.20   $40.20  
  Purchase #2 $127.64   9.732%   $140.0619   $140.06   $140.05   $140.10  
  Purchase #3 $193.97   10.794%   $214.9071   $214.91   $214.90   $214.90  
  Purchase #4 $73.35   10.345%   $80.9381   $80.94   $80.95   $80.90  
  Purchase #5 $136.41   8.661%   $148.2245   $148.22   $148.20   $148.20  
  Purchase #6 $35.28   13.321%   $39.9796   $39.98   $40.00   $40.00  
  Purchase #7 $138.72   7.714%   $149.4209   $149.42   $149.40   $149.40  
  7 purchase totals = $741.07       $813.7442   $813.74   $813.70   $813.70