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 $57.43   13.693%   $65.2939   $65.29   $65.30   $65.30  
  Purchase #2 $74.97   9.366%   $81.9917   $81.99   $82.00   $82.00  
  Purchase #3 $76.99   13.245%   $87.1873   $87.19   $87.20   $87.20  
  Purchase #4 $101.29   7.262%   $108.6457   $108.65   $108.65   $108.60  
  Purchase #5 $56.87   9.379%   $62.2038   $62.20   $62.20   $62.20  
  Purchase #6 $44.60   4.625%   $46.6628   $46.66   $46.65   $46.70  
  Purchase #7 $121.79   13.739%   $138.5227   $138.52   $138.50   $138.50  
  7 purchase totals = $533.94       $590.5079   $590.50   $590.50   $590.50