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 $61.58   8.144%   $66.5951   $66.60   $66.60   $66.60  
  Purchase #2 $80.89   9.507%   $88.5802   $88.58   $88.60   $88.60  
  Purchase #3 $121.67   4.539%   $127.1926   $127.19   $127.20   $127.20  
  Purchase #4 $198.27   8.233%   $214.5936   $214.59   $214.60   $214.60  
  Purchase #5 $44.36   13.812%   $50.487   $50.49   $50.50   $50.50  
  Purchase #6 $208.75   4.779%   $218.7262   $218.73   $218.75   $218.70  
  Purchase #7 $203.40   11.465%   $226.7198   $226.72   $226.70   $226.70  
  7 purchase totals = $918.92       $992.8945   $992.90   $992.95   $992.90