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 $212.60   12.536%   $239.2515   $239.25   $239.25   $239.30  
  Purchase #2 $107.74   3.518%   $111.5303   $111.53   $111.55   $111.50  
  Purchase #3 $68.79   3.642%   $71.2953   $71.30   $71.30   $71.30  
  Purchase #4 $77.27   13.434%   $87.6505   $87.65   $87.65   $87.70  
  Purchase #5 $95.50   6.408%   $101.6196   $101.62   $101.60   $101.60  
  Purchase #6 $55.35   6.652%   $59.0319   $59.03   $59.05   $59.00  
  Purchase #7 $69.66   13.742%   $79.2327   $79.23   $79.25   $79.20  
  7 purchase totals = $686.91       $749.6118   $749.61   $749.65   $749.60