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 $216.72   13.737%   $246.4908   $246.49   $246.50   $246.50  
  Purchase #2 $239.26   12.278%   $268.6363   $268.64   $268.65   $268.60  
  Purchase #3 $17.50   5.248%   $18.4184   $18.42   $18.40   $18.40  
  Purchase #4 $163.94   11.632%   $183.0095   $183.01   $183.00   $183.00  
  Purchase #5 $88.83   12.526%   $99.9568   $99.96   $99.95   $100.00  
  Purchase #6 $188.95   5.799%   $199.9072   $199.91   $199.90   $199.90  
  Purchase #7 $128.82   4.846%   $135.0626   $135.06   $135.05   $135.10  
  7 purchase totals = $1,044.02       $1,151.4816   $1,151.49   $1,151.45   $1,151.50