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 $128.67   3.666%   $133.387   $133.39   $133.40   $133.40  
  Purchase #2 $74.88   5.628%   $79.0942   $79.09   $79.10   $79.10  
  Purchase #3 $89.64   7.559%   $96.4159   $96.42   $96.40   $96.40  
  Purchase #4 $105.87   8.302%   $114.6593   $114.66   $114.65   $114.70  
  Purchase #5 $20.28   3.820%   $21.0547   $21.05   $21.05   $21.10  
  Purchase #6 $197.83   5.736%   $209.1775   $209.18   $209.20   $209.20  
  Purchase #7 $138.80   11.659%   $154.9827   $154.98   $155.00   $155.00  
  7 purchase totals = $755.97       $808.7713   $808.77   $808.80   $808.90