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 $12.50   10.304%   $13.788   $13.79   $13.80   $13.80  
  Purchase #2 $120.93   11.654%   $135.0232   $135.02   $135.00   $135.00  
  Purchase #3 $249.26   11.372%   $277.6058   $277.61   $277.60   $277.60  
  Purchase #4 $219.87   4.114%   $228.9155   $228.92   $228.90   $228.90  
  Purchase #5 $61.27   4.355%   $63.9383   $63.94   $63.95   $63.90  
  Purchase #6 $54.31   4.337%   $56.6654   $56.67   $56.65   $56.70  
  Purchase #7 $185.70   5.605%   $196.1085   $196.11   $196.10   $196.10  
  7 purchase totals = $903.84       $972.0447   $972.06   $972.00   $972.00