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 $83.36   8.449%   $90.4031   $90.40   $90.40   $90.40  
  Purchase #2 $77.76   4.145%   $80.9832   $80.98   $81.00   $81.00  
  Purchase #3 $42.76   6.518%   $45.5471   $45.55   $45.55   $45.50  
  Purchase #4 $19.78   7.768%   $21.3165   $21.32   $21.30   $21.30  
  Purchase #5 $7.67   9.256%   $8.3799   $8.38   $8.40   $8.40  
  Purchase #6 $177.33   11.511%   $197.7425   $197.74   $197.75   $197.70  
  Purchase #7 $138.67   13.445%   $157.3142   $157.31   $157.30   $157.30  
  7 purchase totals = $547.33       $601.6865   $601.68   $601.70   $601.60