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 $30.47   9.170%   $33.2641   $33.26   $33.25   $33.30  
  Purchase #2 $66.27   8.342%   $71.7982   $71.80   $71.80   $71.80  
  Purchase #3 $90.37   5.278%   $95.1397   $95.14   $95.15   $95.10  
  Purchase #4 $49.42   12.561%   $55.6276   $55.63   $55.65   $55.60  
  Purchase #5 $79.83   9.525%   $87.4338   $87.43   $87.45   $87.40  
  Purchase #6 $233.35   5.312%   $245.7456   $245.75   $245.75   $245.70  
  Purchase #7 $47.56   7.356%   $51.0585   $51.06   $51.05   $51.10  
  7 purchase totals = $597.27       $640.0675   $640.07   $640.10   $640.00