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 $194.93   12.121%   $218.5575   $218.56   $218.55   $218.60  
  Purchase #2 $229.81   10.618%   $254.2112   $254.21   $254.20   $254.20  
  Purchase #3 $151.33   4.247%   $157.757   $157.76   $157.75   $157.80  
  Purchase #4 $120.84   13.545%   $137.2078   $137.21   $137.20   $137.20  
  Purchase #5 $60.75   6.293%   $64.573   $64.57   $64.55   $64.60  
  Purchase #6 $35.64   12.779%   $40.1944   $40.19   $40.20   $40.20  
  Purchase #7 $39.42   9.456%   $43.1476   $43.15   $43.15   $43.10  
  7 purchase totals = $832.72       $915.6485   $915.65   $915.60   $915.70