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 $115.70   5.253%   $121.7777   $121.78   $121.80   $121.80  
  Purchase #2 $159.56   8.700%   $173.4417   $173.44   $173.45   $173.40  
  Purchase #3 $139.81   11.525%   $155.9231   $155.92   $155.90   $155.90  
  Purchase #4 $117.37   3.287%   $121.228   $121.23   $121.25   $121.20  
  Purchase #5 $8.72   3.845%   $9.0553   $9.06   $9.05   $9.10  
  Purchase #6 $15.56   3.683%   $16.1331   $16.13   $16.15   $16.10  
  Purchase #7 $129.60   10.628%   $143.3739   $143.37   $143.35   $143.40  
  7 purchase totals = $686.32       $740.9328   $740.93   $740.95   $740.90