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 $74.80   8.679%   $81.2919   $81.29   $81.30   $81.30  
  Purchase #2 $60.71   12.532%   $68.3182   $68.32   $68.30   $68.30  
  Purchase #3 $188.90   9.561%   $206.9607   $206.96   $206.95   $207.00  
  Purchase #4 $133.83   10.532%   $147.925   $147.93   $147.95   $147.90  
  Purchase #5 $192.73   6.544%   $205.3423   $205.34   $205.35   $205.30  
  Purchase #6 $106.32   10.760%   $117.76   $117.76   $117.75   $117.80  
  Purchase #7 $236.94   6.343%   $251.9691   $251.97   $251.95   $252.00  
  7 purchase totals = $994.23       $1,079.5672   $1,079.57   $1,079.55   $1,079.60