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 $20.82   4.566%   $21.7706   $21.77   $21.75   $21.80  
  Purchase #2 $25.51   6.275%   $27.1108   $27.11   $27.10   $27.10  
  Purchase #3 $243.41   6.326%   $258.8081   $258.81   $258.80   $258.80  
  Purchase #4 $140.99   7.299%   $151.2809   $151.28   $151.30   $151.30  
  Purchase #5 $177.95   10.505%   $196.6436   $196.64   $196.65   $196.60  
  Purchase #6 $5.44   12.313%   $6.1098   $6.11   $6.10   $6.10  
  Purchase #7 $167.92   4.232%   $175.0264   $175.03   $175.05   $175.00  
  7 purchase totals = $782.04       $836.7502   $836.75   $836.75   $836.70