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 $19.62   6.568%   $20.9086   $20.91   $20.90   $20.90  
  Purchase #2 $73.28   11.751%   $81.8911   $81.89   $81.90   $81.90  
  Purchase #3 $176.60   9.504%   $193.3841   $193.38   $193.40   $193.40  
  Purchase #4 $182.66   11.555%   $203.7664   $203.77   $203.75   $203.80  
  Purchase #5 $156.55   10.431%   $172.8797   $172.88   $172.90   $172.90  
  Purchase #6 $153.94   8.768%   $167.4375   $167.44   $167.45   $167.40  
  Purchase #7 $79.55   11.637%   $88.8072   $88.81   $88.80   $88.80  
  7 purchase totals = $842.20       $929.0746   $929.08   $929.10   $929.10