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 $223.98   13.758%   $254.7952   $254.80   $254.80   $254.80  
  Purchase #2 $197.59   7.559%   $212.5258   $212.53   $212.55   $212.50  
  Purchase #3 $129.96   3.675%   $134.736   $134.74   $134.75   $134.70  
  Purchase #4 $214.48   5.133%   $225.4893   $225.49   $225.50   $225.50  
  Purchase #5 $198.38   4.768%   $207.8388   $207.84   $207.85   $207.80  
  Purchase #6 $64.87   8.730%   $70.5332   $70.53   $70.55   $70.50  
  Purchase #7 $249.25   6.724%   $266.0096   $266.01   $266.00   $266.00  
  7 purchase totals = $1,278.51       $1,371.9279   $1,371.94   $1,372.00   $1,371.80