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 $92.27   8.685%   $100.2836   $100.28   $100.30   $100.30  
  Purchase #2 $14.92   13.850%   $16.9864   $16.99   $17.00   $17.00  
  Purchase #3 $200.59   4.789%   $210.1963   $210.20   $210.20   $210.20  
  Purchase #4 $31.93   9.182%   $34.8618   $34.86   $34.85   $34.90  
  Purchase #5 $139.37   9.123%   $152.0847   $152.08   $152.10   $152.10  
  Purchase #6 $155.63   3.234%   $160.6631   $160.66   $160.65   $160.70  
  Purchase #7 $8.97   8.294%   $9.714   $9.71   $9.70   $9.70  
  7 purchase totals = $643.68       $684.7899   $684.78   $684.80   $684.90