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 $74.79   12.659%   $84.2577   $84.26   $84.25   $84.30  
  Purchase #2 $111.80   10.578%   $123.6262   $123.63   $123.65   $123.60  
  Purchase #3 $186.56   4.566%   $195.0783   $195.08   $195.10   $195.10  
  Purchase #4 $115.31   13.505%   $130.8826   $130.88   $130.90   $130.90  
  Purchase #5 $227.48   9.265%   $248.556   $248.56   $248.55   $248.60  
  Purchase #6 $176.70   6.434%   $188.0689   $188.07   $188.05   $188.10  
  Purchase #7 $48.29   11.194%   $53.6956   $53.70   $53.70   $53.70  
  7 purchase totals = $940.93       $1,024.1653   $1,024.18   $1,024.20   $1,024.30