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 $156.53   13.372%   $177.4612   $177.46   $177.45   $177.50  
  Purchase #2 $10.41   3.137%   $10.7366   $10.74   $10.75   $10.70  
  Purchase #3 $151.93   11.411%   $169.2667   $169.27   $169.25   $169.30  
  Purchase #4 $236.28   13.817%   $268.9268   $268.93   $268.95   $268.90  
  Purchase #5 $39.71   6.337%   $42.2264   $42.23   $42.25   $42.20  
  Purchase #6 $202.43   13.120%   $228.9888   $228.99   $229.00   $229.00  
  Purchase #7 $86.74   4.593%   $90.724   $90.72   $90.70   $90.70  
  7 purchase totals = $884.03       $988.3305   $988.34   $988.35   $988.30