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 $207.35   6.699%   $221.2404   $221.24   $221.25   $221.20  
  Purchase #2 $96.79   10.170%   $106.6335   $106.63   $106.65   $106.60  
  Purchase #3 $120.75   13.208%   $136.6987   $136.70   $136.70   $136.70  
  Purchase #4 $116.99   7.228%   $125.446   $125.45   $125.45   $125.40  
  Purchase #5 $203.27   9.295%   $222.1639   $222.16   $222.15   $222.20  
  Purchase #6 $93.55   11.454%   $104.2652   $104.27   $104.25   $104.30  
  Purchase #7 $107.67   5.834%   $113.9515   $113.95   $113.95   $114.00  
  7 purchase totals = $946.37       $1,030.3992   $1,030.40   $1,030.40   $1,030.40