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 $27.77   8.319%   $30.0802   $30.08   $30.10   $30.10  
  Purchase #2 $41.97   5.692%   $44.3589   $44.36   $44.35   $44.40  
  Purchase #3 $191.73   5.837%   $202.9213   $202.92   $202.90   $202.90  
  Purchase #4 $170.75   12.475%   $192.0511   $192.05   $192.05   $192.10  
  Purchase #5 $63.25   8.220%   $68.4492   $68.45   $68.45   $68.40  
  Purchase #6 $109.51   10.866%   $121.4094   $121.41   $121.40   $121.40  
  Purchase #7 $40.77   5.254%   $42.9121   $42.91   $42.90   $42.90  
  7 purchase totals = $645.75       $702.1822   $702.18   $702.15   $702.20