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 $122.32   12.150%   $137.1819   $137.18   $137.20   $137.20  
  Purchase #2 $192.66   9.201%   $210.3866   $210.39   $210.40   $210.40  
  Purchase #3 $81.56   9.854%   $89.5969   $89.60   $89.60   $89.60  
  Purchase #4 $14.87   7.182%   $15.938   $15.94   $15.95   $15.90  
  Purchase #5 $36.75   9.281%   $40.1608   $40.16   $40.15   $40.20  
  Purchase #6 $52.96   11.288%   $58.9381   $58.94   $58.95   $58.90  
  Purchase #7 $77.62   13.485%   $88.0871   $88.09   $88.10   $88.10  
  7 purchase totals = $578.74       $640.2894   $640.30   $640.35   $640.30