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 $248.76   3.685%   $257.9268   $257.93   $257.95   $257.90  
  Purchase #2 $105.33   8.541%   $114.3262   $114.33   $114.35   $114.30  
  Purchase #3 $121.81   11.154%   $135.3967   $135.40   $135.40   $135.40  
  Purchase #4 $8.51   13.847%   $9.6884   $9.69   $9.70   $9.70  
  Purchase #5 $77.55   7.697%   $83.519   $83.52   $83.50   $83.50  
  Purchase #6 $222.65   7.841%   $240.108   $240.11   $240.10   $240.10  
  Purchase #7 $148.47   11.418%   $165.4223   $165.42   $165.40   $165.40  
  7 purchase totals = $933.08       $1,006.3874   $1,006.40   $1,006.40   $1,006.30