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 $248.68   10.177%   $273.9882   $273.99   $274.00   $274.00  
  Purchase #2 $57.86   3.758%   $60.0344   $60.03   $60.05   $60.00  
  Purchase #3 $52.79   4.850%   $55.3503   $55.35   $55.35   $55.40  
  Purchase #4 $137.53   8.761%   $149.579   $149.58   $149.60   $149.60  
  Purchase #5 $6.36   3.849%   $6.6048   $6.60   $6.60   $6.60  
  Purchase #6 $179.39   6.525%   $191.0952   $191.10   $191.10   $191.10  
  Purchase #7 $137.67   9.155%   $150.2737   $150.27   $150.25   $150.30  
  7 purchase totals = $820.28       $886.9256   $886.92   $886.95   $887.00