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 $248.94   10.308%   $274.6007   $274.60   $274.60   $274.60  
  Purchase #2 $248.31   12.697%   $279.8379   $279.84   $279.85   $279.80  
  Purchase #3 $109.93   5.807%   $116.3136   $116.31   $116.30   $116.30  
  Purchase #4 $165.53   4.379%   $172.7786   $172.78   $172.80   $172.80  
  Purchase #5 $184.85   9.818%   $202.9986   $203.00   $203.00   $203.00  
  Purchase #6 $71.49   7.159%   $76.608   $76.61   $76.60   $76.60  
  Purchase #7 $118.68   9.413%   $129.8513   $129.85   $129.85   $129.90  
  7 purchase totals = $1,147.73       $1,252.9887   $1,252.99   $1,253.00   $1,253.00