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 $146.37   13.416%   $166.007   $166.01   $166.00   $166.00  
  Purchase #2 $20.47   13.343%   $23.2013   $23.20   $23.20   $23.20  
  Purchase #3 $120.28   12.243%   $135.0059   $135.01   $135.00   $135.00  
  Purchase #4 $213.70   6.122%   $226.7827   $226.78   $226.80   $226.80  
  Purchase #5 $27.95   12.620%   $31.4773   $31.48   $31.50   $31.50  
  Purchase #6 $127.49   5.122%   $134.02   $134.02   $134.00   $134.00  
  Purchase #7 $117.53   8.663%   $127.7116   $127.71   $127.70   $127.70  
  7 purchase totals = $773.79       $844.2058   $844.21   $844.20   $844.20