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 $80.48   9.821%   $88.3839   $88.38   $88.40   $88.40  
  Purchase #2 $233.93   7.108%   $250.5577   $250.56   $250.55   $250.60  
  Purchase #3 $174.51   11.239%   $194.1232   $194.12   $194.10   $194.10  
  Purchase #4 $118.46   12.587%   $133.3706   $133.37   $133.35   $133.40  
  Purchase #5 $11.26   12.702%   $12.6902   $12.69   $12.70   $12.70  
  Purchase #6 $243.29   3.461%   $251.7103   $251.71   $251.70   $251.70  
  Purchase #7 $69.29   5.557%   $73.1404   $73.14   $73.15   $73.10  
  7 purchase totals = $931.22       $1,003.9763   $1,003.97   $1,003.95   $1,004.00