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 $44.53   13.485%   $50.5349   $50.53   $50.55   $50.50  
  Purchase #2 $175.30   9.603%   $192.1341   $192.13   $192.15   $192.10  
  Purchase #3 $179.77   10.291%   $198.2701   $198.27   $198.25   $198.30  
  Purchase #4 $135.92   6.567%   $144.8459   $144.85   $144.85   $144.80  
  Purchase #5 $57.68   8.771%   $62.7391   $62.74   $62.75   $62.70  
  Purchase #6 $228.39   7.272%   $244.9985   $245.00   $245.00   $245.00  
  Purchase #7 $36.71   12.598%   $41.3347   $41.33   $41.35   $41.30  
  7 purchase totals = $858.30       $934.8573   $934.85   $934.90   $934.70