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 $15.75   6.737%   $16.8111   $16.81   $16.80   $16.80  
  Purchase #2 $28.30   12.781%   $31.917   $31.92   $31.90   $31.90  
  Purchase #3 $135.58   7.217%   $145.3648   $145.36   $145.35   $145.40  
  Purchase #4 $122.56   9.314%   $133.9752   $133.98   $134.00   $134.00  
  Purchase #5 $142.38   7.855%   $153.5639   $153.56   $153.55   $153.60  
  Purchase #6 $79.33   3.767%   $82.3184   $82.32   $82.30   $82.30  
  Purchase #7 $116.94   5.605%   $123.4945   $123.49   $123.50   $123.50  
  7 purchase totals = $640.84       $687.4449   $687.44   $687.40   $687.50