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 $26.69   10.587%   $29.5157   $29.52   $29.50   $29.50  
  Purchase #2 $52.44   9.380%   $57.3589   $57.36   $57.35   $57.40  
  Purchase #3 $90.92   13.842%   $103.5051   $103.51   $103.50   $103.50  
  Purchase #4 $108.27   4.692%   $113.35   $113.35   $113.35   $113.40  
  Purchase #5 $61.69   13.395%   $69.9534   $69.95   $69.95   $70.00  
  Purchase #6 $240.56   4.613%   $251.657   $251.66   $251.65   $251.70  
  Purchase #7 $163.58   9.159%   $178.5623   $178.56   $178.55   $178.60  
  7 purchase totals = $744.15       $803.9024   $803.91   $803.85   $804.10