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 $14.34   9.789%   $15.7437   $15.74   $15.75   $15.70  
  Purchase #2 $62.70   7.568%   $67.4451   $67.45   $67.45   $67.40  
  Purchase #3 $8.76   6.712%   $9.348   $9.35   $9.35   $9.30  
  Purchase #4 $127.32   12.614%   $143.3801   $143.38   $143.40   $143.40  
  Purchase #5 $135.85   6.152%   $144.2075   $144.21   $144.20   $144.20  
  Purchase #6 $68.56   3.161%   $70.7272   $70.73   $70.75   $70.70  
  Purchase #7 $171.34   10.385%   $189.1337   $189.13   $189.15   $189.10  
  7 purchase totals = $588.87       $639.9853   $639.99   $640.05   $639.80