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 $12.40   5.783%   $13.1171   $13.12   $13.10   $13.10  
  Purchase #2 $156.64   8.586%   $170.0891   $170.09   $170.10   $170.10  
  Purchase #3 $77.96   13.483%   $88.4713   $88.47   $88.45   $88.50  
  Purchase #4 $247.28   13.569%   $280.8334   $280.83   $280.85   $280.80  
  Purchase #5 $59.84   11.670%   $66.8233   $66.82   $66.80   $66.80  
  Purchase #6 $219.69   11.326%   $244.5721   $244.57   $244.55   $244.60  
  Purchase #7 $63.56   9.731%   $69.745   $69.75   $69.75   $69.70  
  7 purchase totals = $837.37       $933.6513   $933.65   $933.60   $933.60