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 $5.61   13.824%   $6.3855   $6.39   $6.40   $6.40  
  Purchase #2 $169.35   3.110%   $174.6168   $174.62   $174.60   $174.60  
  Purchase #3 $198.51   9.445%   $217.2593   $217.26   $217.25   $217.30  
  Purchase #4 $66.46   7.244%   $71.2744   $71.27   $71.25   $71.30  
  Purchase #5 $81.54   10.629%   $90.2069   $90.21   $90.20   $90.20  
  Purchase #6 $5.95   11.779%   $6.6509   $6.65   $6.65   $6.70  
  Purchase #7 $116.56   6.785%   $124.4686   $124.47   $124.45   $124.50  
  7 purchase totals = $643.98       $690.8624   $690.87   $690.80   $691.00