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 $115.95   4.130%   $120.7387   $120.74   $120.75   $120.70  
  Purchase #2 $82.57   11.230%   $91.8426   $91.84   $91.85   $91.80  
  Purchase #3 $7.28   4.681%   $7.6208   $7.62   $7.60   $7.60  
  Purchase #4 $137.69   7.287%   $147.7235   $147.72   $147.70   $147.70  
  Purchase #5 $36.89   4.288%   $38.4718   $38.47   $38.45   $38.50  
  Purchase #6 $130.54   3.378%   $134.9496   $134.95   $134.95   $134.90  
  Purchase #7 $89.35   10.536%   $98.7639   $98.76   $98.75   $98.80  
  7 purchase totals = $600.27       $640.1109   $640.10   $640.05   $640.00