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 $220.59   13.780%   $250.9873   $250.99   $251.00   $251.00  
  Purchase #2 $166.98   12.147%   $187.2631   $187.26   $187.25   $187.30  
  Purchase #3 $92.80   12.608%   $104.5002   $104.50   $104.50   $104.50  
  Purchase #4 $98.99   10.746%   $109.6275   $109.63   $109.65   $109.60  
  Purchase #5 $209.71   13.159%   $237.3057   $237.31   $237.30   $237.30  
  Purchase #6 $182.91   5.441%   $192.8621   $192.86   $192.85   $192.90  
  Purchase #7 $116.46   5.717%   $123.118   $123.12   $123.10   $123.10  
  7 purchase totals = $1,088.44       $1,205.6639   $1,205.67   $1,205.65   $1,205.70