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 $28.71   10.757%   $31.7983   $31.80   $31.80   $31.80  
  Purchase #2 $135.31   7.339%   $145.2404   $145.24   $145.25   $145.20  
  Purchase #3 $140.44   10.213%   $154.7831   $154.78   $154.80   $154.80  
  Purchase #4 $215.48   13.476%   $244.5181   $244.52   $244.50   $244.50  
  Purchase #5 $11.63   11.732%   $12.9944   $12.99   $13.00   $13.00  
  Purchase #6 $93.95   7.308%   $100.8159   $100.82   $100.80   $100.80  
  Purchase #7 $21.91   10.640%   $24.2412   $24.24   $24.25   $24.20  
  7 purchase totals = $647.43       $714.3914   $714.39   $714.40   $714.30