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 $38.46   9.462%   $42.0991   $42.10   $42.10   $42.10  
  Purchase #2 $77.63   4.112%   $80.8221   $80.82   $80.80   $80.80  
  Purchase #3 $237.40   13.673%   $269.8597   $269.86   $269.85   $269.90  
  Purchase #4 $182.45   10.570%   $201.735   $201.74   $201.75   $201.70  
  Purchase #5 $172.37   11.501%   $192.1943   $192.19   $192.20   $192.20  
  Purchase #6 $114.98   9.446%   $125.841   $125.84   $125.85   $125.80  
  Purchase #7 $123.46   4.136%   $128.5663   $128.57   $128.55   $128.60  
  7 purchase totals = $946.75       $1,041.1175   $1,041.12   $1,041.10   $1,041.10