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 $121.48   7.152%   $130.1682   $130.17   $130.15   $130.20  
  Purchase #2 $128.54   5.404%   $135.4863   $135.49   $135.50   $135.50  
  Purchase #3 $27.78   3.403%   $28.7254   $28.73   $28.75   $28.70  
  Purchase #4 $243.88   11.553%   $272.0555   $272.06   $272.05   $272.10  
  Purchase #5 $248.80   10.774%   $275.6057   $275.61   $275.60   $275.60  
  Purchase #6 $61.93   7.555%   $66.6088   $66.61   $66.60   $66.60  
  Purchase #7 $11.61   3.714%   $12.0412   $12.04   $12.05   $12.00  
  7 purchase totals = $844.02       $920.6911   $920.71   $920.70   $920.70