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 $36.76   9.174%   $40.1324   $40.13   $40.15   $40.10  
  Purchase #2 $72.74   9.660%   $79.7667   $79.77   $79.75   $79.80  
  Purchase #3 $109.69   7.713%   $118.1504   $118.15   $118.15   $118.20  
  Purchase #4 $221.39   11.688%   $247.2661   $247.27   $247.25   $247.30  
  Purchase #5 $11.35   10.561%   $12.5487   $12.55   $12.55   $12.50  
  Purchase #6 $6.66   9.797%   $7.3125   $7.31   $7.30   $7.30  
  Purchase #7 $130.25   6.395%   $138.5795   $138.58   $138.60   $138.60  
  7 purchase totals = $588.84       $643.7563   $643.76   $643.75   $643.80