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 $248.99   8.536%   $270.2438   $270.24   $270.25   $270.20  
  Purchase #2 $147.98   12.184%   $166.0099   $166.01   $166.00   $166.00  
  Purchase #3 $53.25   6.110%   $56.5036   $56.50   $56.50   $56.50  
  Purchase #4 $214.82   6.428%   $228.6286   $228.63   $228.65   $228.60  
  Purchase #5 $65.44   6.261%   $69.5372   $69.54   $69.55   $69.50  
  Purchase #6 $124.97   7.678%   $134.5652   $134.57   $134.55   $134.60  
  Purchase #7 $37.86   8.795%   $41.1898   $41.19   $41.20   $41.20  
  7 purchase totals = $893.31       $966.6781   $966.68   $966.70   $966.60