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 $50.54   6.214%   $53.6806   $53.68   $53.70   $53.70  
  Purchase #2 $249.25   10.250%   $274.7981   $274.80   $274.80   $274.80  
  Purchase #3 $205.53   4.341%   $214.4521   $214.45   $214.45   $214.50  
  Purchase #4 $137.56   3.109%   $141.8367   $141.84   $141.85   $141.80  
  Purchase #5 $13.60   8.460%   $14.7506   $14.75   $14.75   $14.80  
  Purchase #6 $52.75   4.266%   $55.0003   $55.00   $55.00   $55.00  
  Purchase #7 $231.49   5.402%   $243.9951   $244.00   $244.00   $244.00  
  7 purchase totals = $940.72       $998.5135   $998.52   $998.55   $998.60