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 $239.38   10.661%   $264.9003   $264.90   $264.90   $264.90  
  Purchase #2 $210.77   9.465%   $230.7194   $230.72   $230.70   $230.70  
  Purchase #3 $71.41   10.682%   $79.038   $79.04   $79.05   $79.00  
  Purchase #4 $73.50   5.778%   $77.7468   $77.75   $77.75   $77.70  
  Purchase #5 $214.75   3.215%   $221.6542   $221.65   $221.65   $221.70  
  Purchase #6 $135.46   3.443%   $140.1239   $140.12   $140.10   $140.10  
  Purchase #7 $15.76   8.599%   $17.1152   $17.12   $17.10   $17.10  
  7 purchase totals = $961.03       $1,031.2978   $1,031.30   $1,031.25   $1,031.20