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 $18.90   10.712%   $20.9246   $20.92   $20.90   $20.90  
  Purchase #2 $114.90   12.615%   $129.3946   $129.39   $129.40   $129.40  
  Purchase #3 $145.52   6.580%   $155.0952   $155.10   $155.10   $155.10  
  Purchase #4 $174.60   9.579%   $191.3249   $191.32   $191.30   $191.30  
  Purchase #5 $176.56   7.837%   $190.397   $190.40   $190.40   $190.40  
  Purchase #6 $13.53   12.187%   $15.1789   $15.18   $15.20   $15.20  
  Purchase #7 $217.64   12.175%   $244.1377   $244.14   $244.15   $244.10  
  7 purchase totals = $861.65       $946.4529   $946.45   $946.45   $946.40