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 $45.86   8.629%   $49.8173   $49.82   $49.80   $49.80  
  Purchase #2 $61.51   10.369%   $67.888   $67.89   $67.90   $67.90  
  Purchase #3 $214.72   11.576%   $239.576   $239.58   $239.60   $239.60  
  Purchase #4 $184.67   3.614%   $191.344   $191.34   $191.35   $191.30  
  Purchase #5 $178.67   11.788%   $199.7316   $199.73   $199.75   $199.70  
  Purchase #6 $216.29   7.146%   $231.7461   $231.75   $231.75   $231.70  
  Purchase #7 $249.43   8.562%   $270.7862   $270.79   $270.80   $270.80  
  7 purchase totals = $1,151.15       $1,250.8892   $1,250.90   $1,250.95   $1,250.80