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 $17.30   9.121%   $18.8779   $18.88   $18.90   $18.90  
  Purchase #2 $126.68   8.545%   $137.5048   $137.50   $137.50   $137.50  
  Purchase #3 $26.89   11.522%   $29.9883   $29.99   $30.00   $30.00  
  Purchase #4 $29.57   6.392%   $31.4601   $31.46   $31.45   $31.50  
  Purchase #5 $92.48   12.615%   $104.1464   $104.15   $104.15   $104.10  
  Purchase #6 $173.45   13.135%   $196.2327   $196.23   $196.25   $196.20  
  Purchase #7 $54.62   12.790%   $61.6059   $61.61   $61.60   $61.60  
  7 purchase totals = $520.99       $579.8161   $579.82   $579.85   $579.80