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 $19.92   4.645%   $20.8453   $20.85   $20.85   $20.80  
  Purchase #2 $76.30   3.121%   $78.6813   $78.68   $78.70   $78.70  
  Purchase #3 $21.79   9.267%   $23.8093   $23.81   $23.80   $23.80  
  Purchase #4 $29.64   12.342%   $33.2982   $33.30   $33.30   $33.30  
  Purchase #5 $191.32   10.778%   $211.9405   $211.94   $211.95   $211.90  
  Purchase #6 $32.28   13.789%   $36.7311   $36.73   $36.75   $36.70  
  Purchase #7 $183.80   6.403%   $195.5687   $195.57   $195.55   $195.60  
  7 purchase totals = $555.05       $600.8744   $600.88   $600.90   $600.80