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 $12.75   9.189%   $13.9216   $13.92   $13.90   $13.90  
  Purchase #2 $200.51   8.808%   $218.1709   $218.17   $218.15   $218.20  
  Purchase #3 $97.85   5.382%   $103.1163   $103.12   $103.10   $103.10  
  Purchase #4 $161.80   9.586%   $177.3101   $177.31   $177.30   $177.30  
  Purchase #5 $164.71   6.570%   $175.5314   $175.53   $175.55   $175.50  
  Purchase #6 $82.73   11.623%   $92.3457   $92.35   $92.35   $92.30  
  Purchase #7 $197.89   10.187%   $218.0491   $218.05   $218.05   $218.00  
  7 purchase totals = $918.24       $998.4451   $998.45   $998.40   $998.30