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 $122.71   6.731%   $130.9696   $130.97   $130.95   $131.00  
  Purchase #2 $145.43   12.121%   $163.0576   $163.06   $163.05   $163.10  
  Purchase #3 $209.36   7.642%   $225.3593   $225.36   $225.35   $225.40  
  Purchase #4 $138.51   5.276%   $145.8178   $145.82   $145.80   $145.80  
  Purchase #5 $50.51   9.118%   $55.1155   $55.12   $55.10   $55.10  
  Purchase #6 $32.30   7.183%   $34.6201   $34.62   $34.60   $34.60  
  Purchase #7 $150.40   10.149%   $165.6641   $165.66   $165.65   $165.70  
  7 purchase totals = $849.22       $920.6040   $920.61   $920.50   $920.70