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 $20.40   4.550%   $21.3282   $21.33   $21.35   $21.30  
  Purchase #2 $140.32   7.212%   $150.4399   $150.44   $150.45   $150.40  
  Purchase #3 $144.75   7.381%   $155.434   $155.43   $155.45   $155.40  
  Purchase #4 $168.89   4.784%   $176.9697   $176.97   $176.95   $177.00  
  Purchase #5 $123.94   7.446%   $133.1686   $133.17   $133.15   $133.20  
  Purchase #6 $250.51   10.457%   $276.7058   $276.71   $276.70   $276.70  
  Purchase #7 $79.45   6.534%   $84.6413   $84.64   $84.65   $84.60  
  7 purchase totals = $928.26       $998.6875   $998.69   $998.70   $998.60