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 $238.62   12.469%   $268.3735   $268.37   $268.35   $268.40  
  Purchase #2 $19.37   12.415%   $21.7748   $21.77   $21.75   $21.80  
  Purchase #3 $126.73   6.536%   $135.0131   $135.01   $135.00   $135.00  
  Purchase #4 $228.38   5.839%   $241.7151   $241.72   $241.70   $241.70  
  Purchase #5 $246.41   6.280%   $261.8845   $261.88   $261.90   $261.90  
  Purchase #6 $131.78   11.776%   $147.2984   $147.30   $147.30   $147.30  
  Purchase #7 $174.77   3.699%   $181.2347   $181.23   $181.25   $181.20  
  7 purchase totals = $1,166.06       $1,257.2941   $1,257.28   $1,257.25   $1,257.30