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 $141.53   12.337%   $158.9906   $158.99   $159.00   $159.00  
  Purchase #2 $107.53   3.714%   $111.5237   $111.52   $111.50   $111.50  
  Purchase #3 $9.70   10.111%   $10.6808   $10.68   $10.70   $10.70  
  Purchase #4 $15.38   13.261%   $17.4195   $17.42   $17.40   $17.40  
  Purchase #5 $22.40   3.720%   $23.2333   $23.23   $23.25   $23.20  
  Purchase #6 $174.79   7.583%   $188.0443   $188.04   $188.05   $188.00  
  Purchase #7 $220.59   3.608%   $228.5489   $228.55   $228.55   $228.50  
  7 purchase totals = $691.92       $738.4411   $738.43   $738.45   $738.30