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 $45.40   13.211%   $51.3978   $51.40   $51.40   $51.40  
  Purchase #2 $6.69   12.349%   $7.5161   $7.52   $7.50   $7.50  
  Purchase #3 $99.39   7.735%   $107.0778   $107.08   $107.10   $107.10  
  Purchase #4 $186.64   12.745%   $210.4273   $210.43   $210.45   $210.40  
  Purchase #5 $34.49   6.351%   $36.6805   $36.68   $36.70   $36.70  
  Purchase #6 $45.94   5.540%   $48.4851   $48.49   $48.50   $48.50  
  Purchase #7 $66.85   13.705%   $76.0118   $76.01   $76.00   $76.00  
  7 purchase totals = $485.40       $537.5964   $537.61   $537.65   $537.60