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 $18.88   9.835%   $20.7368   $20.74   $20.75   $20.70  
  Purchase #2 $172.79   7.301%   $185.4054   $185.41   $185.40   $185.40  
  Purchase #3 $195.74   10.199%   $215.7035   $215.70   $215.70   $215.70  
  Purchase #4 $207.31   10.762%   $229.6207   $229.62   $229.60   $229.60  
  Purchase #5 $230.76   7.652%   $248.4178   $248.42   $248.40   $248.40  
  Purchase #6 $223.41   8.752%   $242.9628   $242.96   $242.95   $243.00  
  Purchase #7 $181.32   3.675%   $187.9835   $187.98   $188.00   $188.00  
  7 purchase totals = $1,230.21       $1,330.8305   $1,330.83   $1,330.80   $1,330.80