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 $123.73   12.580%   $139.2952   $139.30   $139.30   $139.30  
  Purchase #2 $188.96   6.811%   $201.8301   $201.83   $201.85   $201.80  
  Purchase #3 $33.71   3.492%   $34.8872   $34.89   $34.90   $34.90  
  Purchase #4 $10.76   6.515%   $11.461   $11.46   $11.45   $11.50  
  Purchase #5 $239.49   12.589%   $269.6394   $269.64   $269.65   $269.60  
  Purchase #6 $180.74   7.307%   $193.9467   $193.95   $193.95   $193.90  
  Purchase #7 $248.85   13.638%   $282.7882   $282.79   $282.80   $282.80  
  7 purchase totals = $1,026.24       $1,133.8478   $1,133.86   $1,133.90   $1,133.80