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 $249.32   9.812%   $273.7833   $273.78   $273.80   $273.80  
  Purchase #2 $130.29   3.743%   $135.1668   $135.17   $135.15   $135.20  
  Purchase #3 $102.76   3.117%   $105.963   $105.96   $105.95   $106.00  
  Purchase #4 $168.98   8.683%   $183.6525   $183.65   $183.65   $183.70  
  Purchase #5 $150.64   6.451%   $160.3578   $160.36   $160.35   $160.40  
  Purchase #6 $231.59   9.639%   $253.913   $253.91   $253.90   $253.90  
  Purchase #7 $248.48   4.774%   $260.3424   $260.34   $260.35   $260.30  
  7 purchase totals = $1,282.06       $1,373.1788   $1,373.17   $1,373.15   $1,373.30