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 $151.54   3.365%   $156.6393   $156.64   $156.65   $156.60  
  Purchase #2 $106.97   13.375%   $121.2772   $121.28   $121.30   $121.30  
  Purchase #3 $12.29   6.617%   $13.1032   $13.10   $13.10   $13.10  
  Purchase #4 $216.60   7.113%   $232.0068   $232.01   $232.00   $232.00  
  Purchase #5 $113.32   6.331%   $120.4943   $120.49   $120.50   $120.50  
  Purchase #6 $144.67   3.284%   $149.421   $149.42   $149.40   $149.40  
  Purchase #7 $62.37   3.160%   $64.3409   $64.34   $64.35   $64.30  
  7 purchase totals = $807.76       $857.2827   $857.28   $857.30   $857.20