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 $227.33   5.146%   $239.0284   $239.03   $239.05   $239.00  
  Purchase #2 $17.70   10.773%   $19.6068   $19.61   $19.60   $19.60  
  Purchase #3 $44.61   11.874%   $49.907   $49.91   $49.90   $49.90  
  Purchase #4 $118.83   8.503%   $128.9341   $128.93   $128.95   $128.90  
  Purchase #5 $227.48   7.471%   $244.475   $244.48   $244.50   $244.50  
  Purchase #6 $88.39   8.807%   $96.1745   $96.17   $96.15   $96.20  
  Purchase #7 $37.75   11.199%   $41.9776   $41.98   $42.00   $42.00  
  7 purchase totals = $762.09       $820.1034   $820.11   $820.15   $820.10