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 $239.98   6.652%   $255.9435   $255.94   $255.95   $255.90  
  Purchase #2 $53.34   6.506%   $56.8103   $56.81   $56.80   $56.80  
  Purchase #3 $55.60   12.679%   $62.6495   $62.65   $62.65   $62.60  
  Purchase #4 $220.89   6.384%   $234.9916   $234.99   $235.00   $235.00  
  Purchase #5 $195.51   3.292%   $201.9462   $201.95   $201.95   $201.90  
  Purchase #6 $197.71   8.182%   $213.8866   $213.89   $213.90   $213.90  
  Purchase #7 $5.97   11.834%   $6.6765   $6.68   $6.70   $6.70  
  7 purchase totals = $969.00       $1,032.9042   $1,032.91   $1,032.95   $1,032.80