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 $238.90   13.369%   $270.8385   $270.84   $270.85   $270.80  
  Purchase #2 $94.56   13.679%   $107.4949   $107.49   $107.50   $107.50  
  Purchase #3 $143.51   11.808%   $160.4557   $160.46   $160.45   $160.50  
  Purchase #4 $12.80   7.847%   $13.8044   $13.80   $13.80   $13.80  
  Purchase #5 $180.61   4.118%   $188.0475   $188.05   $188.05   $188.00  
  Purchase #6 $171.44   10.495%   $189.4326   $189.43   $189.45   $189.40  
  Purchase #7 $11.68   4.809%   $12.2417   $12.24   $12.25   $12.20  
  7 purchase totals = $853.50       $942.3153   $942.31   $942.35   $942.20