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 $250.47   6.508%   $266.7706   $266.77   $266.75   $266.80  
  Purchase #2 $167.29   11.351%   $186.2791   $186.28   $186.30   $186.30  
  Purchase #3 $130.70   8.764%   $142.1545   $142.15   $142.15   $142.20  
  Purchase #4 $240.64   11.547%   $268.4267   $268.43   $268.45   $268.40  
  Purchase #5 $59.42   10.244%   $65.507   $65.51   $65.50   $65.50  
  Purchase #6 $236.25   7.143%   $253.1253   $253.13   $253.15   $253.10  
  Purchase #7 $219.69   3.697%   $227.8119   $227.81   $227.80   $227.80  
  7 purchase totals = $1,304.46       $1,410.0751   $1,410.08   $1,410.10   $1,410.10