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 $17.58   7.640%   $18.9231   $18.92   $18.90   $18.90  
  Purchase #2 $184.61   3.626%   $191.304   $191.30   $191.30   $191.30  
  Purchase #3 $119.30   11.407%   $132.9086   $132.91   $132.90   $132.90  
  Purchase #4 $28.68   12.397%   $32.2355   $32.24   $32.25   $32.20  
  Purchase #5 $133.77   10.605%   $147.9563   $147.96   $147.95   $148.00  
  Purchase #6 $120.92   13.473%   $137.2116   $137.21   $137.20   $137.20  
  Purchase #7 $231.85   12.500%   $260.8313   $260.83   $260.85   $260.80  
  7 purchase totals = $836.71       $921.3704   $921.37   $921.35   $921.30