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 $5.69   12.460%   $6.399   $6.40   $6.40   $6.40  
  Purchase #2 $11.31   8.600%   $12.2827   $12.28   $12.30   $12.30  
  Purchase #3 $186.56   13.659%   $212.0422   $212.04   $212.05   $212.00  
  Purchase #4 $128.43   3.455%   $132.8673   $132.87   $132.85   $132.90  
  Purchase #5 $143.88   10.829%   $159.4608   $159.46   $159.45   $159.50  
  Purchase #6 $152.93   4.322%   $159.5396   $159.54   $159.55   $159.50  
  Purchase #7 $170.71   11.132%   $189.7134   $189.71   $189.70   $189.70  
  7 purchase totals = $799.51       $872.3050   $872.30   $872.30   $872.30