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 $7.55   12.301%   $8.4787   $8.48   $8.50   $8.50  
  Purchase #2 $133.54   6.318%   $141.9771   $141.98   $142.00   $142.00  
  Purchase #3 $75.81   3.280%   $78.2966   $78.30   $78.30   $78.30  
  Purchase #4 $130.90   4.403%   $136.6635   $136.66   $136.65   $136.70  
  Purchase #5 $83.35   9.251%   $91.0607   $91.06   $91.05   $91.10  
  Purchase #6 $26.31   6.762%   $28.0891   $28.09   $28.10   $28.10  
  Purchase #7 $30.79   12.559%   $34.6569   $34.66   $34.65   $34.70  
  7 purchase totals = $488.25       $519.2226   $519.23   $519.25   $519.40