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 $19.69   3.777%   $20.4337   $20.43   $20.45   $20.40  
  Purchase #2 $170.68   8.257%   $184.773   $184.77   $184.75   $184.80  
  Purchase #3 $126.64   8.668%   $137.6172   $137.62   $137.60   $137.60  
  Purchase #4 $26.96   13.324%   $30.5522   $30.55   $30.55   $30.60  
  Purchase #5 $135.60   3.500%   $140.346   $140.35   $140.35   $140.30  
  Purchase #6 $76.48   12.186%   $85.7999   $85.80   $85.80   $85.80  
  Purchase #7 $60.56   9.210%   $66.1376   $66.14   $66.15   $66.10  
  7 purchase totals = $616.61       $665.6596   $665.66   $665.65   $665.60