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 $100.65   6.115%   $106.8047   $106.80   $106.80   $106.80  
  Purchase #2 $19.53   9.765%   $21.4371   $21.44   $21.45   $21.40  
  Purchase #3 $27.44   8.431%   $29.7535   $29.75   $29.75   $29.80  
  Purchase #4 $114.86   6.418%   $122.2317   $122.23   $122.25   $122.20  
  Purchase #5 $98.35   7.408%   $105.6358   $105.64   $105.65   $105.60  
  Purchase #6 $27.36   7.840%   $29.505   $29.51   $29.50   $29.50  
  Purchase #7 $21.94   4.594%   $22.9479   $22.95   $22.95   $22.90  
  7 purchase totals = $410.13       $438.3157   $438.32   $438.35   $438.20