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 $157.74   4.365%   $164.6254   $164.63   $164.65   $164.60  
  Purchase #2 $168.63   11.854%   $188.6194   $188.62   $188.60   $188.60  
  Purchase #3 $137.76   5.590%   $145.4608   $145.46   $145.45   $145.50  
  Purchase #4 $248.61   9.738%   $272.8196   $272.82   $272.80   $272.80  
  Purchase #5 $30.27   12.214%   $33.9672   $33.97   $33.95   $34.00  
  Purchase #6 $29.72   8.451%   $32.2316   $32.23   $32.25   $32.20  
  Purchase #7 $74.98   11.592%   $83.6717   $83.67   $83.65   $83.70  
  7 purchase totals = $847.71       $921.3957   $921.40   $921.35   $921.40