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 $129.59   4.842%   $135.8647   $135.86   $135.85   $135.90  
  Purchase #2 $44.69   10.789%   $49.5116   $49.51   $49.50   $49.50  
  Purchase #3 $72.76   11.324%   $80.9993   $81.00   $81.00   $81.00  
  Purchase #4 $28.61   11.731%   $31.9662   $31.97   $31.95   $32.00  
  Purchase #5 $149.28   8.664%   $162.2136   $162.21   $162.20   $162.20  
  Purchase #6 $36.97   7.795%   $39.8518   $39.85   $39.85   $39.90  
  Purchase #7 $94.62   4.226%   $98.6186   $98.62   $98.60   $98.60  
  7 purchase totals = $556.52       $599.0258   $599.02   $598.95   $599.10