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 $133.42   3.782%   $138.4659   $138.47   $138.45   $138.50  
  Purchase #2 $68.31   9.784%   $74.9935   $74.99   $75.00   $75.00  
  Purchase #3 $146.42   5.629%   $154.662   $154.66   $154.65   $154.70  
  Purchase #4 $10.76   3.144%   $11.0983   $11.10   $11.10   $11.10  
  Purchase #5 $21.31   10.253%   $23.4949   $23.49   $23.50   $23.50  
  Purchase #6 $38.60   8.389%   $41.8382   $41.84   $41.85   $41.80  
  Purchase #7 $178.64   10.783%   $197.9028   $197.90   $197.90   $197.90  
  7 purchase totals = $597.46       $642.4556   $642.45   $642.45   $642.50