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 $120.90   12.451%   $135.9533   $135.95   $135.95   $136.00  
  Purchase #2 $59.37   10.606%   $65.6668   $65.67   $65.65   $65.70  
  Purchase #3 $104.47   11.565%   $116.552   $116.55   $116.55   $116.60  
  Purchase #4 $15.92   12.350%   $17.8861   $17.89   $17.90   $17.90  
  Purchase #5 $23.25   8.712%   $25.2755   $25.28   $25.30   $25.30  
  Purchase #6 $22.35   12.423%   $25.1265   $25.13   $25.15   $25.10  
  Purchase #7 $226.76   6.251%   $240.9348   $240.93   $240.95   $240.90  
  7 purchase totals = $573.02       $627.3950   $627.40   $627.45   $627.50