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 $16.61   7.581%   $17.8692   $17.87   $17.85   $17.90  
  Purchase #2 $121.25   10.184%   $133.5981   $133.60   $133.60   $133.60  
  Purchase #3 $198.92   12.114%   $223.0172   $223.02   $223.00   $223.00  
  Purchase #4 $209.98   12.113%   $235.4149   $235.41   $235.40   $235.40  
  Purchase #5 $33.58   12.511%   $37.7812   $37.78   $37.80   $37.80  
  Purchase #6 $89.33   6.449%   $95.0909   $95.09   $95.10   $95.10  
  Purchase #7 $204.42   11.216%   $227.3477   $227.35   $227.35   $227.30  
  7 purchase totals = $874.09       $970.1192   $970.12   $970.10   $970.10