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 $233.86   13.460%   $265.3376   $265.34   $265.35   $265.30  
  Purchase #2 $125.67   4.160%   $130.8979   $130.90   $130.90   $130.90  
  Purchase #3 $19.64   3.602%   $20.3474   $20.35   $20.35   $20.30  
  Purchase #4 $124.74   7.874%   $134.562   $134.56   $134.55   $134.60  
  Purchase #5 $39.67   12.393%   $44.5863   $44.59   $44.60   $44.60  
  Purchase #6 $127.34   6.711%   $135.8858   $135.89   $135.90   $135.90  
  Purchase #7 $250.51   6.542%   $266.8984   $266.90   $266.90   $266.90  
  7 purchase totals = $921.43       $998.5154   $998.53   $998.55   $998.50