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 $19.65   8.239%   $21.269   $21.27   $21.25   $21.30  
  Purchase #2 $108.96   5.425%   $114.8711   $114.87   $114.85   $114.90  
  Purchase #3 $176.27   12.692%   $198.6422   $198.64   $198.65   $198.60  
  Purchase #4 $132.49   13.272%   $150.0741   $150.07   $150.05   $150.10  
  Purchase #5 $74.94   3.610%   $77.6453   $77.65   $77.65   $77.60  
  Purchase #6 $53.81   6.468%   $57.2904   $57.29   $57.30   $57.30  
  Purchase #7 $33.64   12.692%   $37.9096   $37.91   $37.90   $37.90  
  7 purchase totals = $599.76       $657.7017   $657.70   $657.65   $657.70