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 $17.49   8.263%   $18.9352   $18.94   $18.95   $18.90  
  Purchase #2 $139.32   3.312%   $143.9343   $143.93   $143.95   $143.90  
  Purchase #3 $108.81   10.482%   $120.2155   $120.22   $120.20   $120.20  
  Purchase #4 $31.53   5.585%   $33.291   $33.29   $33.30   $33.30  
  Purchase #5 $249.44   8.378%   $270.3381   $270.34   $270.35   $270.30  
  Purchase #6 $181.47   9.787%   $199.2305   $199.23   $199.25   $199.20  
  Purchase #7 $183.94   10.248%   $202.7902   $202.79   $202.80   $202.80  
  7 purchase totals = $912.00       $988.7348   $988.74   $988.80   $988.60