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 $36.36   8.768%   $39.548   $39.55   $39.55   $39.50  
  Purchase #2 $88.68   10.500%   $97.9914   $97.99   $98.00   $98.00  
  Purchase #3 $46.52   10.171%   $51.2515   $51.25   $51.25   $51.30  
  Purchase #4 $162.65   5.197%   $171.1029   $171.10   $171.10   $171.10  
  Purchase #5 $135.44   12.305%   $152.1059   $152.11   $152.10   $152.10  
  Purchase #6 $67.82   9.647%   $74.3626   $74.36   $74.35   $74.40  
  Purchase #7 $47.82   11.700%   $53.4149   $53.41   $53.40   $53.40  
  7 purchase totals = $585.29       $639.7772   $639.77   $639.75   $639.80