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 $119.39   13.620%   $135.6509   $135.65   $135.65   $135.70  
  Purchase #2 $59.56   9.495%   $65.2152   $65.22   $65.20   $65.20  
  Purchase #3 $249.88   3.606%   $258.8907   $258.89   $258.90   $258.90  
  Purchase #4 $93.66   11.431%   $104.3663   $104.37   $104.35   $104.40  
  Purchase #5 $129.27   7.359%   $138.783   $138.78   $138.80   $138.80  
  Purchase #6 $43.85   4.595%   $45.8649   $45.86   $45.85   $45.90  
  Purchase #7 $172.52   9.395%   $188.7283   $188.73   $188.75   $188.70  
  7 purchase totals = $868.13       $937.4993   $937.50   $937.50   $937.60