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 $48.35   12.286%   $54.2903   $54.29   $54.30   $54.30  
  Purchase #2 $201.99   10.285%   $222.7647   $222.76   $222.75   $222.80  
  Purchase #3 $109.40   10.310%   $120.6791   $120.68   $120.70   $120.70  
  Purchase #4 $208.87   8.292%   $226.1895   $226.19   $226.20   $226.20  
  Purchase #5 $70.39   5.871%   $74.5226   $74.52   $74.50   $74.50  
  Purchase #6 $11.57   12.276%   $12.9903   $12.99   $13.00   $13.00  
  Purchase #7 $244.67   5.622%   $258.4253   $258.43   $258.45   $258.40  
  7 purchase totals = $895.24       $969.8618   $969.86   $969.90   $969.90