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 $90.87   8.392%   $98.4958   $98.50   $98.50   $98.50  
  Purchase #2 $163.82   3.121%   $168.9328   $168.93   $168.95   $168.90  
  Purchase #3 $55.64   3.679%   $57.687   $57.69   $57.70   $57.70  
  Purchase #4 $83.91   4.541%   $87.7204   $87.72   $87.70   $87.70  
  Purchase #5 $129.71   3.543%   $134.3056   $134.31   $134.30   $134.30  
  Purchase #6 $23.71   3.209%   $24.4709   $24.47   $24.45   $24.50  
  Purchase #7 $20.52   3.835%   $21.3069   $21.31   $21.30   $21.30  
  7 purchase totals = $568.18       $592.9194   $592.93   $592.90   $592.90