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 $35.69   13.639%   $40.5578   $40.56   $40.55   $40.60  
  Purchase #2 $229.82   9.228%   $251.0278   $251.03   $251.05   $251.00  
  Purchase #3 $179.51   5.196%   $188.8373   $188.84   $188.85   $188.80  
  Purchase #4 $6.72   6.307%   $7.1438   $7.14   $7.15   $7.10  
  Purchase #5 $24.53   4.454%   $25.6226   $25.62   $25.60   $25.60  
  Purchase #6 $177.87   7.214%   $190.7015   $190.70   $190.70   $190.70  
  Purchase #7 $128.61   5.854%   $136.1388   $136.14   $136.15   $136.10  
  7 purchase totals = $782.75       $840.0296   $840.03   $840.05   $839.90