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 $20.99   5.783%   $22.2039   $22.20   $22.20   $22.20  
  Purchase #2 $134.84   6.732%   $143.9174   $143.92   $143.90   $143.90  
  Purchase #3 $189.67   12.544%   $213.4622   $213.46   $213.45   $213.50  
  Purchase #4 $89.70   10.423%   $99.0494   $99.05   $99.05   $99.00  
  Purchase #5 $10.55   9.699%   $11.5732   $11.57   $11.55   $11.60  
  Purchase #6 $54.79   4.586%   $57.3027   $57.30   $57.30   $57.30  
  Purchase #7 $11.53   9.241%   $12.5955   $12.60   $12.60   $12.60  
  7 purchase totals = $512.07       $560.1043   $560.10   $560.05   $560.10