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 $121.69   5.471%   $128.3477   $128.35   $128.35   $128.30  
  Purchase #2 $83.42   13.281%   $94.499   $94.50   $94.50   $94.50  
  Purchase #3 $58.74   7.391%   $63.0815   $63.08   $63.10   $63.10  
  Purchase #4 $110.77   9.265%   $121.0328   $121.03   $121.05   $121.00  
  Purchase #5 $32.56   9.668%   $35.7079   $35.71   $35.70   $35.70  
  Purchase #6 $45.98   10.534%   $50.8235   $50.82   $50.80   $50.80  
  Purchase #7 $130.84   13.231%   $148.1514   $148.15   $148.15   $148.20  
  7 purchase totals = $584.00       $641.6438   $641.64   $641.65   $641.60