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 $27.75   3.450%   $28.7074   $28.71   $28.70   $28.70  
  Purchase #2 $128.49   7.352%   $137.9366   $137.94   $137.95   $137.90  
  Purchase #3 $79.26   4.403%   $82.7498   $82.75   $82.75   $82.70  
  Purchase #4 $68.31   10.155%   $75.2469   $75.25   $75.25   $75.20  
  Purchase #5 $88.68   11.866%   $99.2028   $99.20   $99.20   $99.20  
  Purchase #6 $113.90   11.263%   $126.7286   $126.73   $126.75   $126.70  
  Purchase #7 $79.74   8.794%   $86.7523   $86.75   $86.75   $86.80  
  7 purchase totals = $586.13       $637.3244   $637.33   $637.35   $637.20