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 $108.53   13.218%   $122.8755   $122.88   $122.90   $122.90  
  Purchase #2 $166.70   10.318%   $183.9001   $183.90   $183.90   $183.90  
  Purchase #3 $162.35   8.690%   $176.4582   $176.46   $176.45   $176.50  
  Purchase #4 $111.68   9.815%   $122.6414   $122.64   $122.65   $122.60  
  Purchase #5 $171.82   10.563%   $189.9693   $189.97   $189.95   $190.00  
  Purchase #6 $98.73   9.213%   $107.826   $107.83   $107.85   $107.80  
  Purchase #7 $138.35   11.322%   $154.014   $154.01   $154.00   $154.00  
  7 purchase totals = $958.16       $1,057.6845   $1,057.69   $1,057.70   $1,057.70