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 $190.66   9.612%   $208.9862   $208.99   $209.00   $209.00  
  Purchase #2 $113.65   11.453%   $126.6663   $126.67   $126.65   $126.70  
  Purchase #3 $147.68   13.225%   $167.2107   $167.21   $167.20   $167.20  
  Purchase #4 $77.35   4.383%   $80.7403   $80.74   $80.75   $80.70  
  Purchase #5 $86.32   5.306%   $90.9001   $90.90   $90.90   $90.90  
  Purchase #6 $68.30   11.703%   $76.2931   $76.29   $76.30   $76.30  
  Purchase #7 $199.76   3.288%   $206.3281   $206.33   $206.35   $206.30  
  7 purchase totals = $883.72       $957.1248   $957.13   $957.15   $957.10