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 $20.69   4.222%   $21.5635   $21.56   $21.55   $21.60  
  Purchase #2 $195.70   12.628%   $220.413   $220.41   $220.40   $220.40  
  Purchase #3 $215.51   10.112%   $237.3024   $237.30   $237.30   $237.30  
  Purchase #4 $110.96   12.383%   $124.7002   $124.70   $124.70   $124.70  
  Purchase #5 $192.32   7.254%   $206.2709   $206.27   $206.25   $206.30  
  Purchase #6 $248.97   6.305%   $264.6676   $264.67   $264.65   $264.70  
  Purchase #7 $223.79   5.827%   $236.8302   $236.83   $236.85   $236.80  
  7 purchase totals = $1,207.94       $1,311.7478   $1,311.74   $1,311.70   $1,311.80