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 $235.59   10.782%   $260.9913   $260.99   $261.00   $261.00  
  Purchase #2 $166.69   13.193%   $188.6814   $188.68   $188.70   $188.70  
  Purchase #3 $145.26   6.608%   $154.8588   $154.86   $154.85   $154.90  
  Purchase #4 $204.33   12.577%   $230.0286   $230.03   $230.05   $230.00  
  Purchase #5 $41.61   8.831%   $45.2846   $45.28   $45.30   $45.30  
  Purchase #6 $10.48   13.846%   $11.9311   $11.93   $11.95   $11.90  
  Purchase #7 $17.77   12.787%   $20.0422   $20.04   $20.05   $20.00  
  7 purchase totals = $821.73       $911.8180   $911.81   $911.90   $911.80