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 $238.44   12.839%   $269.0533   $269.05   $269.05   $269.10  
  Purchase #2 $205.85   6.179%   $218.5695   $218.57   $218.55   $218.60  
  Purchase #3 $33.51   9.586%   $36.7223   $36.72   $36.70   $36.70  
  Purchase #4 $99.91   7.290%   $107.1934   $107.19   $107.20   $107.20  
  Purchase #5 $17.36   5.167%   $18.257   $18.26   $18.25   $18.30  
  Purchase #6 $6.27   4.726%   $6.5663   $6.57   $6.55   $6.60  
  Purchase #7 $6.64   7.570%   $7.1426   $7.14   $7.15   $7.10  
  7 purchase totals = $607.98       $663.5044   $663.50   $663.45   $663.60