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 $61.70   5.849%   $65.3088   $65.31   $65.30   $65.30  
  Purchase #2 $203.25   11.149%   $225.9103   $225.91   $225.90   $225.90  
  Purchase #3 $124.88   10.302%   $137.7451   $137.75   $137.75   $137.70  
  Purchase #4 $106.77   9.831%   $117.2666   $117.27   $117.25   $117.30  
  Purchase #5 $73.79   6.727%   $78.7539   $78.75   $78.75   $78.80  
  Purchase #6 $32.99   12.301%   $37.0481   $37.05   $37.05   $37.00  
  Purchase #7 $23.94   12.270%   $26.8774   $26.88   $26.90   $26.90  
  7 purchase totals = $627.32       $688.9102   $688.92   $688.90   $688.90