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 $28.39   7.178%   $30.4278   $30.43   $30.45   $30.40  
  Purchase #2 $19.82   5.448%   $20.8998   $20.90   $20.90   $20.90  
  Purchase #3 $121.84   11.136%   $135.4081   $135.41   $135.40   $135.40  
  Purchase #4 $179.39   5.673%   $189.5668   $189.57   $189.55   $189.60  
  Purchase #5 $94.96   4.265%   $99.01   $99.01   $99.00   $99.00  
  Purchase #6 $11.29   9.742%   $12.3899   $12.39   $12.40   $12.40  
  Purchase #7 $40.63   9.701%   $44.5715   $44.57   $44.55   $44.60  
  7 purchase totals = $496.32       $532.2739   $532.28   $532.25   $532.30