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 $123.30   12.140%   $138.2686   $138.27   $138.25   $138.30  
  Purchase #2 $139.59   6.868%   $149.177   $149.18   $149.20   $149.20  
  Purchase #3 $193.92   4.193%   $202.0511   $202.05   $202.05   $202.10  
  Purchase #4 $20.90   10.742%   $23.1451   $23.15   $23.15   $23.10  
  Purchase #5 $7.79   7.476%   $8.3724   $8.37   $8.35   $8.40  
  Purchase #6 $119.57   3.538%   $123.8004   $123.80   $123.80   $123.80  
  Purchase #7 $42.51   6.363%   $45.2149   $45.21   $45.20   $45.20  
  7 purchase totals = $647.58       $690.0295   $690.03   $690.00   $690.10