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 $120.50   13.616%   $136.9073   $136.91   $136.90   $136.90  
  Purchase #2 $230.43   8.204%   $249.3345   $249.33   $249.35   $249.30  
  Purchase #3 $71.85   12.298%   $80.6861   $80.69   $80.70   $80.70  
  Purchase #4 $194.53   12.446%   $218.7412   $218.74   $218.75   $218.70  
  Purchase #5 $187.86   4.506%   $196.325   $196.33   $196.35   $196.30  
  Purchase #6 $159.49   12.481%   $179.3959   $179.40   $179.40   $179.40  
  Purchase #7 $166.36   11.790%   $185.9738   $185.97   $185.95   $186.00  
  7 purchase totals = $1,131.02       $1,247.3638   $1,247.37   $1,247.40   $1,247.30