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 $30.40   3.479%   $31.4576   $31.46   $31.45   $31.50  
  Purchase #2 $165.97   4.760%   $173.8702   $173.87   $173.85   $173.90  
  Purchase #3 $187.65   11.612%   $209.4399   $209.44   $209.45   $209.40  
  Purchase #4 $214.73   5.683%   $226.9331   $226.93   $226.95   $226.90  
  Purchase #5 $106.35   8.658%   $115.5578   $115.56   $115.55   $115.60  
  Purchase #6 $113.79   8.375%   $123.3199   $123.32   $123.30   $123.30  
  Purchase #7 $88.43   12.824%   $99.7703   $99.77   $99.75   $99.80  
  7 purchase totals = $907.32       $980.3488   $980.35   $980.30   $980.40