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 $220.67   12.152%   $247.4858   $247.49   $247.50   $247.50  
  Purchase #2 $180.98   13.161%   $204.7988   $204.80   $204.80   $204.80  
  Purchase #3 $135.77   7.212%   $145.5617   $145.56   $145.55   $145.60  
  Purchase #4 $33.76   5.747%   $35.7002   $35.70   $35.70   $35.70  
  Purchase #5 $63.70   10.606%   $70.456   $70.46   $70.45   $70.50  
  Purchase #6 $207.30   11.794%   $231.749   $231.75   $231.75   $231.70  
  Purchase #7 $120.57   7.762%   $129.9286   $129.93   $129.95   $129.90  
  7 purchase totals = $962.75       $1,065.6801   $1,065.69   $1,065.70   $1,065.70