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 $249.50   13.807%   $283.9485   $283.95   $283.95   $283.90  
  Purchase #2 $211.99   12.449%   $238.3806   $238.38   $238.40   $238.40  
  Purchase #3 $59.89   13.667%   $68.0752   $68.08   $68.10   $68.10  
  Purchase #4 $213.84   5.676%   $225.9776   $225.98   $226.00   $226.00  
  Purchase #5 $242.66   8.786%   $263.9801   $263.98   $264.00   $264.00  
  Purchase #6 $142.35   9.597%   $156.0113   $156.01   $156.00   $156.00  
  Purchase #7 $44.54   13.451%   $50.5311   $50.53   $50.55   $50.50  
  7 purchase totals = $1,164.77       $1,286.9044   $1,286.91   $1,287.00   $1,286.90