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 $39.40   8.719%   $42.8353   $42.84   $42.85   $42.80  
  Purchase #2 $157.63   9.177%   $172.0957   $172.10   $172.10   $172.10  
  Purchase #3 $126.51   6.336%   $134.5257   $134.53   $134.55   $134.50  
  Purchase #4 $17.39   12.107%   $19.4954   $19.50   $19.50   $19.50  
  Purchase #5 $31.53   6.501%   $33.5798   $33.58   $33.60   $33.60  
  Purchase #6 $217.88   4.644%   $227.9983   $228.00   $228.00   $228.00  
  Purchase #7 $6.45   7.233%   $6.9165   $6.92   $6.90   $6.90  
  7 purchase totals = $596.79       $637.4467   $637.47   $637.50   $637.40