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 $242.39   13.537%   $275.2023   $275.20   $275.20   $275.20  
  Purchase #2 $231.90   12.504%   $260.8968   $260.90   $260.90   $260.90  
  Purchase #3 $62.31   11.764%   $69.6401   $69.64   $69.65   $69.60  
  Purchase #4 $146.60   8.704%   $159.3601   $159.36   $159.35   $159.40  
  Purchase #5 $208.83   6.808%   $223.0471   $223.05   $223.05   $223.00  
  Purchase #6 $40.27   3.197%   $41.5574   $41.56   $41.55   $41.60  
  Purchase #7 $224.78   8.782%   $244.5202   $244.52   $244.50   $244.50  
  7 purchase totals = $1,157.08       $1,274.2240   $1,274.23   $1,274.20   $1,274.20