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 $34.68   12.206%   $38.913   $38.91   $38.90   $38.90  
  Purchase #2 $169.66   3.646%   $175.8458   $175.85   $175.85   $175.80  
  Purchase #3 $21.47   3.565%   $22.2354   $22.24   $22.25   $22.20  
  Purchase #4 $59.38   5.213%   $62.4755   $62.48   $62.50   $62.50  
  Purchase #5 $247.41   9.704%   $271.4187   $271.42   $271.40   $271.40  
  Purchase #6 $123.60   9.485%   $135.3235   $135.32   $135.30   $135.30  
  Purchase #7 $181.32   10.614%   $200.5653   $200.57   $200.55   $200.60  
  7 purchase totals = $837.52       $906.7772   $906.79   $906.75   $906.70