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 $121.98   10.337%   $134.5891   $134.59   $134.60   $134.60  
  Purchase #2 $150.95   3.775%   $156.6484   $156.65   $156.65   $156.60  
  Purchase #3 $62.69   4.763%   $65.6759   $65.68   $65.70   $65.70  
  Purchase #4 $200.81   9.153%   $219.1901   $219.19   $219.20   $219.20  
  Purchase #5 $173.74   11.441%   $193.6176   $193.62   $193.60   $193.60  
  Purchase #6 $15.95   10.502%   $17.6251   $17.63   $17.65   $17.60  
  Purchase #7 $183.54   4.754%   $192.2655   $192.27   $192.25   $192.30  
  7 purchase totals = $909.66       $979.6117   $979.63   $979.65   $979.60