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 $91.31   8.438%   $99.0147   $99.01   $99.00   $99.00  
  Purchase #2 $192.89   10.754%   $213.6334   $213.63   $213.65   $213.60  
  Purchase #3 $5.30   12.563%   $5.9658   $5.97   $5.95   $6.00  
  Purchase #4 $215.28   12.215%   $241.5765   $241.58   $241.60   $241.60  
  Purchase #5 $236.67   6.425%   $251.876   $251.88   $251.90   $251.90  
  Purchase #6 $162.29   3.709%   $168.3093   $168.31   $168.30   $168.30  
  Purchase #7 $231.69   9.679%   $254.1153   $254.12   $254.10   $254.10  
  7 purchase totals = $1,135.43       $1,234.4910   $1,234.50   $1,234.50   $1,234.50