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 $232.62   12.190%   $260.9764   $260.98   $261.00   $261.00  
  Purchase #2 $22.49   13.653%   $25.5606   $25.56   $25.55   $25.60  
  Purchase #3 $113.65   5.698%   $120.1258   $120.13   $120.15   $120.10  
  Purchase #4 $92.96   11.851%   $103.9767   $103.98   $104.00   $104.00  
  Purchase #5 $240.40   13.754%   $273.4646   $273.46   $273.45   $273.50  
  Purchase #6 $50.94   8.820%   $55.4329   $55.43   $55.45   $55.40  
  Purchase #7 $70.35   9.463%   $77.0072   $77.01   $77.00   $77.00  
  7 purchase totals = $823.41       $916.5442   $916.55   $916.60   $916.60