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 $28.58   8.448%   $30.9944   $30.99   $31.00   $31.00  
  Purchase #2 $250.88   9.387%   $274.4301   $274.43   $274.45   $274.40  
  Purchase #3 $163.96   8.177%   $177.367   $177.37   $177.35   $177.40  
  Purchase #4 $219.32   5.199%   $230.7224   $230.72   $230.70   $230.70  
  Purchase #5 $73.37   7.157%   $78.6211   $78.62   $78.60   $78.60  
  Purchase #6 $28.81   10.130%   $31.7285   $31.73   $31.75   $31.70  
  Purchase #7 $166.94   13.792%   $189.9644   $189.96   $189.95   $190.00  
  7 purchase totals = $931.86       $1,013.8279   $1,013.82   $1,013.80   $1,013.80