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 $6.59   11.625%   $7.3561   $7.36   $7.35   $7.40  
  Purchase #2 $22.44   4.540%   $23.4588   $23.46   $23.45   $23.50  
  Purchase #3 $162.47   6.467%   $172.9769   $172.98   $173.00   $173.00  
  Purchase #4 $149.76   4.768%   $156.9006   $156.90   $156.90   $156.90  
  Purchase #5 $250.38   8.717%   $272.2056   $272.21   $272.20   $272.20  
  Purchase #6 $13.74   7.749%   $14.8047   $14.80   $14.80   $14.80  
  Purchase #7 $15.38   9.856%   $16.8959   $16.90   $16.90   $16.90  
  7 purchase totals = $620.76       $664.5986   $664.61   $664.60   $664.70