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 $241.84   12.202%   $271.3493   $271.35   $271.35   $271.30  
  Purchase #2 $47.94   8.742%   $52.1309   $52.13   $52.15   $52.10  
  Purchase #3 $5.83   5.633%   $6.1584   $6.16   $6.15   $6.20  
  Purchase #4 $182.93   10.556%   $202.2401   $202.24   $202.25   $202.20  
  Purchase #5 $241.84   10.831%   $268.0337   $268.03   $268.05   $268.00  
  Purchase #6 $97.47   6.232%   $103.5443   $103.54   $103.55   $103.50  
  Purchase #7 $32.31   3.642%   $33.4867   $33.49   $33.50   $33.50  
  7 purchase totals = $850.16       $936.9434   $936.94   $937.00   $936.80