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 $19.43   12.398%   $21.8389   $21.84   $21.85   $21.80  
  Purchase #2 $111.91   7.571%   $120.3827   $120.38   $120.40   $120.40  
  Purchase #3 $22.76   3.674%   $23.5962   $23.60   $23.60   $23.60  
  Purchase #4 $228.84   8.377%   $248.0099   $248.01   $248.00   $248.00  
  Purchase #5 $81.95   3.171%   $84.5486   $84.55   $84.55   $84.50  
  Purchase #6 $145.47   5.133%   $152.937   $152.94   $152.95   $152.90  
  Purchase #7 $77.25   13.514%   $87.6896   $87.69   $87.70   $87.70  
  7 purchase totals = $687.61       $739.0029   $739.01   $739.05   $738.90