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 $159.61   13.142%   $180.5859   $180.59   $180.60   $180.60  
  Purchase #2 $214.41   12.279%   $240.7374   $240.74   $240.75   $240.70  
  Purchase #3 $144.76   12.462%   $162.8   $162.80   $162.80   $162.80  
  Purchase #4 $80.45   8.852%   $87.5714   $87.57   $87.55   $87.60  
  Purchase #5 $28.57   4.196%   $29.7688   $29.77   $29.75   $29.80  
  Purchase #6 $70.35   3.294%   $72.6673   $72.67   $72.65   $72.70  
  Purchase #7 $231.39   6.353%   $246.0902   $246.09   $246.10   $246.10  
  7 purchase totals = $929.54       $1,020.2210   $1,020.23   $1,020.20   $1,020.30