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 $114.67   6.229%   $121.8128   $121.81   $121.80   $121.80  
  Purchase #2 $81.74   3.285%   $84.4252   $84.43   $84.45   $84.40  
  Purchase #3 $203.33   8.121%   $219.8424   $219.84   $219.85   $219.80  
  Purchase #4 $143.57   9.789%   $157.6241   $157.62   $157.60   $157.60  
  Purchase #5 $214.52   5.509%   $226.3379   $226.34   $226.35   $226.30  
  Purchase #6 $154.34   5.644%   $163.0509   $163.05   $163.05   $163.10  
  Purchase #7 $231.99   8.324%   $251.3008   $251.30   $251.30   $251.30  
  7 purchase totals = $1,144.16       $1,224.3941   $1,224.39   $1,224.40   $1,224.30