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 $20.48   4.233%   $21.3469   $21.35   $21.35   $21.30  
  Purchase #2 $143.57   13.651%   $163.1687   $163.17   $163.15   $163.20  
  Purchase #3 $190.90   9.278%   $208.6117   $208.61   $208.60   $208.60  
  Purchase #4 $5.46   7.175%   $5.8518   $5.85   $5.85   $5.90  
  Purchase #5 $144.84   3.185%   $149.4532   $149.45   $149.45   $149.50  
  Purchase #6 $20.29   3.494%   $20.9989   $21.00   $21.00   $21.00  
  Purchase #7 $64.94   12.839%   $73.2776   $73.28   $73.30   $73.30  
  7 purchase totals = $590.48       $642.7088   $642.71   $642.70   $642.80