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 $250.94   11.252%   $279.1758   $279.18   $279.20   $279.20  
  Purchase #2 $76.36   11.748%   $85.3308   $85.33   $85.35   $85.30  
  Purchase #3 $158.79   9.550%   $173.9544   $173.95   $173.95   $174.00  
  Purchase #4 $98.55   6.158%   $104.6187   $104.62   $104.60   $104.60  
  Purchase #5 $116.58   9.596%   $127.767   $127.77   $127.75   $127.80  
  Purchase #6 $227.64   6.397%   $242.2021   $242.20   $242.20   $242.20  
  Purchase #7 $7.56   13.583%   $8.5869   $8.59   $8.60   $8.60  
  7 purchase totals = $936.42       $1,021.6357   $1,021.64   $1,021.65   $1,021.70