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 $183.61   3.442%   $189.9299   $189.93   $189.95   $189.90  
  Purchase #2 $71.89   12.859%   $81.1343   $81.13   $81.15   $81.10  
  Purchase #3 $163.40   4.337%   $170.4867   $170.49   $170.50   $170.50  
  Purchase #4 $99.63   10.685%   $110.2755   $110.28   $110.30   $110.30  
  Purchase #5 $68.59   9.648%   $75.2076   $75.21   $75.20   $75.20  
  Purchase #6 $38.58   4.177%   $40.1915   $40.19   $40.20   $40.20  
  Purchase #7 $12.51   3.632%   $12.9644   $12.96   $12.95   $13.00  
  7 purchase totals = $638.21       $680.1899   $680.19   $680.25   $680.20