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 $204.32   12.571%   $230.0051   $230.01   $230.00   $230.00  
  Purchase #2 $138.84   6.748%   $148.2089   $148.21   $148.20   $148.20  
  Purchase #3 $137.90   6.856%   $147.3544   $147.35   $147.35   $147.40  
  Purchase #4 $250.92   10.544%   $277.377   $277.38   $277.40   $277.40  
  Purchase #5 $193.79   6.706%   $206.7856   $206.79   $206.80   $206.80  
  Purchase #6 $229.86   6.103%   $243.8884   $243.89   $243.90   $243.90  
  Purchase #7 $63.53   11.147%   $70.6117   $70.61   $70.60   $70.60  
  7 purchase totals = $1,219.16       $1,324.2311   $1,324.24   $1,324.25   $1,324.30