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 $106.61   3.325%   $110.1548   $110.15   $110.15   $110.20  
  Purchase #2 $71.60   11.507%   $79.839   $79.84   $79.85   $79.80  
  Purchase #3 $121.32   10.781%   $134.3995   $134.40   $134.40   $134.40  
  Purchase #4 $78.98   11.805%   $88.3036   $88.30   $88.30   $88.30  
  Purchase #5 $29.87   11.484%   $33.3003   $33.30   $33.30   $33.30  
  Purchase #6 $172.44   3.380%   $178.2685   $178.27   $178.25   $178.30  
  Purchase #7 $14.51   11.317%   $16.1521   $16.15   $16.15   $16.20  
  7 purchase totals = $595.33       $640.4178   $640.41   $640.40   $640.50