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 $12.36   8.861%   $13.4552   $13.46   $13.45   $13.50  
  Purchase #2 $238.72   8.338%   $258.6245   $258.62   $258.60   $258.60  
  Purchase #3 $12.61   4.676%   $13.1996   $13.20   $13.20   $13.20  
  Purchase #4 $43.28   4.430%   $45.1973   $45.20   $45.20   $45.20  
  Purchase #5 $185.93   9.240%   $203.1099   $203.11   $203.10   $203.10  
  Purchase #6 $106.71   13.493%   $121.1084   $121.11   $121.10   $121.10  
  Purchase #7 $28.59   11.193%   $31.7901   $31.79   $31.80   $31.80  
  7 purchase totals = $628.20       $686.4850   $686.49   $686.45   $686.50