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 $63.35   8.205%   $68.5479   $68.55   $68.55   $68.50  
  Purchase #2 $243.69   9.115%   $265.9023   $265.90   $265.90   $265.90  
  Purchase #3 $175.37   4.325%   $182.9548   $182.95   $182.95   $183.00  
  Purchase #4 $61.62   11.137%   $68.4826   $68.48   $68.50   $68.50  
  Purchase #5 $62.84   4.458%   $65.6414   $65.64   $65.65   $65.60  
  Purchase #6 $67.89   5.484%   $71.6131   $71.61   $71.60   $71.60  
  Purchase #7 $87.39   8.586%   $94.8933   $94.89   $94.90   $94.90  
  7 purchase totals = $762.15       $818.0354   $818.02   $818.05   $818.00