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 $227.51   10.683%   $251.8149   $251.81   $251.80   $251.80  
  Purchase #2 $180.54   12.242%   $202.6417   $202.64   $202.65   $202.60  
  Purchase #3 $112.77   13.228%   $127.6872   $127.69   $127.70   $127.70  
  Purchase #4 $222.61   4.663%   $232.9903   $232.99   $233.00   $233.00  
  Purchase #5 $241.83   3.265%   $249.7257   $249.73   $249.75   $249.70  
  Purchase #6 $26.47   7.727%   $28.5153   $28.52   $28.50   $28.50  
  Purchase #7 $105.63   13.621%   $120.0179   $120.02   $120.00   $120.00  
  7 purchase totals = $1,117.36       $1,213.3930   $1,213.40   $1,213.40   $1,213.30