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 $112.73   5.713%   $119.1703   $119.17   $119.15   $119.20  
  Purchase #2 $48.57   12.631%   $54.7049   $54.70   $54.70   $54.70  
  Purchase #3 $161.84   6.608%   $172.5344   $172.53   $172.55   $172.50  
  Purchase #4 $144.60   12.729%   $163.0061   $163.01   $163.00   $163.00  
  Purchase #5 $242.93   4.713%   $254.3793   $254.38   $254.40   $254.40  
  Purchase #6 $64.91   8.259%   $70.2709   $70.27   $70.25   $70.30  
  Purchase #7 $5.46   8.227%   $5.9092   $5.91   $5.90   $5.90  
  7 purchase totals = $781.04       $839.9751   $839.97   $839.95   $840.00