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 $139.71   5.741%   $147.7308   $147.73   $147.75   $147.70  
  Purchase #2 $77.59   12.245%   $87.0909   $87.09   $87.10   $87.10  
  Purchase #3 $64.91   9.714%   $71.2154   $71.22   $71.20   $71.20  
  Purchase #4 $59.28   3.383%   $61.2854   $61.29   $61.30   $61.30  
  Purchase #5 $159.38   12.620%   $179.4938   $179.49   $179.50   $179.50  
  Purchase #6 $154.98   4.182%   $161.4613   $161.46   $161.45   $161.50  
  Purchase #7 $126.57   5.196%   $133.1466   $133.15   $133.15   $133.10  
  7 purchase totals = $782.42       $841.4242   $841.43   $841.45   $841.40