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 $192.34   6.156%   $204.1805   $204.18   $204.20   $204.20  
  Purchase #2 $173.98   9.811%   $191.0492   $191.05   $191.05   $191.00  
  Purchase #3 $54.64   5.359%   $57.5682   $57.57   $57.55   $57.60  
  Purchase #4 $48.57   8.414%   $52.6567   $52.66   $52.65   $52.70  
  Purchase #5 $102.44   3.231%   $105.7498   $105.75   $105.75   $105.70  
  Purchase #6 $232.57   3.531%   $240.782   $240.78   $240.80   $240.80  
  Purchase #7 $160.53   5.667%   $169.6272   $169.63   $169.65   $169.60  
  7 purchase totals = $965.07       $1,021.6136   $1,021.62   $1,021.65   $1,021.60