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 $19.28   8.826%   $20.9817   $20.98   $21.00   $21.00  
  Purchase #2 $239.83   12.647%   $270.1613   $270.16   $270.15   $270.20  
  Purchase #3 $171.86   10.131%   $189.2711   $189.27   $189.25   $189.30  
  Purchase #4 $191.99   10.114%   $211.4079   $211.41   $211.40   $211.40  
  Purchase #5 $172.40   7.144%   $184.7163   $184.72   $184.70   $184.70  
  Purchase #6 $67.40   3.300%   $69.6242   $69.62   $69.60   $69.60  
  Purchase #7 $213.34   8.332%   $231.1155   $231.12   $231.10   $231.10  
  7 purchase totals = $1,076.10       $1,177.2780   $1,177.28   $1,177.20   $1,177.30