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 $129.63   4.114%   $134.963   $134.96   $134.95   $135.00  
  Purchase #2 $20.96   13.635%   $23.8179   $23.82   $23.80   $23.80  
  Purchase #3 $127.64   6.579%   $136.0374   $136.04   $136.05   $136.00  
  Purchase #4 $95.54   6.824%   $102.0596   $102.06   $102.05   $102.10  
  Purchase #5 $206.70   4.249%   $215.4827   $215.48   $215.50   $215.50  
  Purchase #6 $178.66   7.829%   $192.6473   $192.65   $192.65   $192.60  
  Purchase #7 $117.25   3.859%   $121.7747   $121.77   $121.75   $121.80  
  7 purchase totals = $876.38       $926.7826   $926.78   $926.75   $926.80