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 $20.50   3.123%   $21.1402   $21.14   $21.15   $21.10  
  Purchase #2 $12.81   5.282%   $13.4866   $13.49   $13.50   $13.50  
  Purchase #3 $234.25   12.547%   $263.6413   $263.64   $263.65   $263.60  
  Purchase #4 $137.65   7.773%   $148.3495   $148.35   $148.35   $148.30  
  Purchase #5 $36.83   9.211%   $40.2224   $40.22   $40.20   $40.20  
  Purchase #6 $59.91   6.822%   $63.9971   $64.00   $64.00   $64.00  
  Purchase #7 $109.39   9.319%   $119.5841   $119.58   $119.60   $119.60  
  7 purchase totals = $611.34       $670.4212   $670.42   $670.45   $670.30