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 $27.99   3.526%   $28.9769   $28.98   $29.00   $29.00  
  Purchase #2 $229.81   7.842%   $247.8317   $247.83   $247.85   $247.80  
  Purchase #3 $144.41   10.837%   $160.0597   $160.06   $160.05   $160.10  
  Purchase #4 $16.54   3.441%   $17.1091   $17.11   $17.10   $17.10  
  Purchase #5 $189.30   3.417%   $195.7684   $195.77   $195.75   $195.80  
  Purchase #6 $107.67   11.592%   $120.1511   $120.15   $120.15   $120.20  
  Purchase #7 $171.37   10.518%   $189.3947   $189.39   $189.40   $189.40  
  7 purchase totals = $887.09       $959.2916   $959.29   $959.30   $959.40