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 $122.55   13.466%   $139.0526   $139.05   $139.05   $139.10  
  Purchase #2 $23.36   3.207%   $24.1092   $24.11   $24.10   $24.10  
  Purchase #3 $227.78   11.110%   $253.0864   $253.09   $253.10   $253.10  
  Purchase #4 $165.66   13.457%   $187.9529   $187.95   $187.95   $188.00  
  Purchase #5 $107.80   10.183%   $118.7773   $118.78   $118.80   $118.80  
  Purchase #6 $233.76   13.374%   $265.0231   $265.02   $265.00   $265.00  
  Purchase #7 $105.53   3.504%   $109.2278   $109.23   $109.25   $109.20  
  7 purchase totals = $986.44       $1,097.2293   $1,097.23   $1,097.25   $1,097.30