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 $9.40   6.173%   $9.9803   $9.98   $10.00   $10.00  
  Purchase #2 $136.59   3.315%   $141.118   $141.12   $141.10   $141.10  
  Purchase #3 $25.29   10.786%   $28.0178   $28.02   $28.00   $28.00  
  Purchase #4 $230.51   9.620%   $252.6851   $252.69   $252.70   $252.70  
  Purchase #5 $36.76   6.493%   $39.1468   $39.15   $39.15   $39.10  
  Purchase #6 $68.76   10.796%   $76.1833   $76.18   $76.20   $76.20  
  Purchase #7 $67.86   7.195%   $72.7425   $72.74   $72.75   $72.70  
  7 purchase totals = $575.17       $619.8738   $619.88   $619.90   $619.80