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 $170.39   5.594%   $179.9216   $179.92   $179.90   $179.90  
  Purchase #2 $115.80   4.331%   $120.8153   $120.82   $120.80   $120.80  
  Purchase #3 $123.93   13.494%   $140.6531   $140.65   $140.65   $140.70  
  Purchase #4 $168.72   5.294%   $177.652   $177.65   $177.65   $177.70  
  Purchase #5 $219.83   5.161%   $231.1754   $231.18   $231.20   $231.20  
  Purchase #6 $33.82   13.345%   $38.3333   $38.33   $38.35   $38.30  
  Purchase #7 $18.38   10.123%   $20.2406   $20.24   $20.25   $20.20  
  7 purchase totals = $850.87       $908.7913   $908.79   $908.80   $908.80