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 $120.40   7.863%   $129.8671   $129.87   $129.85   $129.90  
  Purchase #2 $70.81   6.716%   $75.5656   $75.57   $75.55   $75.60  
  Purchase #3 $230.55   9.489%   $252.4269   $252.43   $252.45   $252.40  
  Purchase #4 $213.56   11.230%   $237.5428   $237.54   $237.55   $237.50  
  Purchase #5 $66.89   3.537%   $69.2559   $69.26   $69.25   $69.30  
  Purchase #6 $128.43   6.716%   $137.0554   $137.06   $137.05   $137.10  
  Purchase #7 $19.63   9.801%   $21.5539   $21.55   $21.55   $21.60  
  7 purchase totals = $850.27       $923.2676   $923.28   $923.25   $923.40