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 $180.85   12.766%   $203.9373   $203.94   $203.95   $203.90  
  Purchase #2 $174.76   9.690%   $191.6942   $191.69   $191.70   $191.70  
  Purchase #3 $89.91   11.384%   $100.1454   $100.15   $100.15   $100.10  
  Purchase #4 $152.38   4.536%   $159.292   $159.29   $159.30   $159.30  
  Purchase #5 $169.36   8.410%   $183.6032   $183.60   $183.60   $183.60  
  Purchase #6 $108.92   9.508%   $119.2761   $119.28   $119.30   $119.30  
  Purchase #7 $67.41   13.207%   $76.3128   $76.31   $76.30   $76.30  
  7 purchase totals = $943.59       $1,034.2610   $1,034.26   $1,034.30   $1,034.20