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 $246.82   13.490%   $280.116   $280.12   $280.10   $280.10  
  Purchase #2 $208.97   12.873%   $235.8707   $235.87   $235.85   $235.90  
  Purchase #3 $134.93   9.158%   $147.2869   $147.29   $147.30   $147.30  
  Purchase #4 $116.79   13.679%   $132.7657   $132.77   $132.75   $132.80  
  Purchase #5 $86.60   12.304%   $97.2553   $97.26   $97.25   $97.30  
  Purchase #6 $98.60   8.836%   $107.3123   $107.31   $107.30   $107.30  
  Purchase #7 $236.78   7.715%   $255.0476   $255.05   $255.05   $255.00  
  7 purchase totals = $1,129.49       $1,255.6545   $1,255.67   $1,255.60   $1,255.70