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 $120.49   12.634%   $135.7127   $135.71   $135.70   $135.70  
  Purchase #2 $18.68   12.401%   $20.9965   $21.00   $21.00   $21.00  
  Purchase #3 $138.42   5.550%   $146.1023   $146.10   $146.10   $146.10  
  Purchase #4 $90.69   7.363%   $97.3675   $97.37   $97.35   $97.40  
  Purchase #5 $74.27   11.484%   $82.7992   $82.80   $82.80   $82.80  
  Purchase #6 $130.27   7.868%   $140.5196   $140.52   $140.50   $140.50  
  Purchase #7 $23.49   3.322%   $24.2703   $24.27   $24.25   $24.30  
  7 purchase totals = $596.31       $647.7681   $647.77   $647.70   $647.80