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 $242.30   6.483%   $258.0083   $258.01   $258.00   $258.00  
  Purchase #2 $169.80   8.285%   $183.8679   $183.87   $183.85   $183.90  
  Purchase #3 $85.39   7.657%   $91.9283   $91.93   $91.95   $91.90  
  Purchase #4 $138.63   11.253%   $154.23   $154.23   $154.25   $154.20  
  Purchase #5 $157.65   6.351%   $167.6624   $167.66   $167.65   $167.70  
  Purchase #6 $248.90   6.170%   $264.2571   $264.26   $264.25   $264.30  
  Purchase #7 $152.70   8.694%   $165.9757   $165.98   $166.00   $166.00  
  7 purchase totals = $1,195.37       $1,285.9297   $1,285.94   $1,285.95   $1,286.00