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 $242.99   10.304%   $268.0277   $268.03   $268.05   $268.00  
  Purchase #2 $57.85   5.496%   $61.0294   $61.03   $61.05   $61.00  
  Purchase #3 $243.82   6.816%   $260.4388   $260.44   $260.45   $260.40  
  Purchase #4 $165.56   12.591%   $186.4057   $186.41   $186.40   $186.40  
  Purchase #5 $43.61   12.282%   $48.9662   $48.97   $48.95   $49.00  
  Purchase #6 $33.84   5.441%   $35.6812   $35.68   $35.70   $35.70  
  Purchase #7 $66.58   6.666%   $71.0182   $71.02   $71.00   $71.00  
  7 purchase totals = $854.25       $931.5672   $931.58   $931.60   $931.50